API docs (#379)

* Move API documentation into separate directory

* Rearrange main docs bar

* Split python examples into separate page

* Fix broken links

* Adds page for browseable API

* Fix image links

* Add entrypoint page for barcodes

* Update barcode docs

* Add (empty) pages for internal and external barcods

* Add documentation on "internal" barcode format

* Documentation for external barcode functionality

* Skeleton page for custom barcode information

* Extend docs for custom barcodesplugins

* Add stubs for new API docs

* Add documentation for downloading data via the AP

* API metadata information

* docs for pythonic metadata access

* docs for bulk delete

docs/barcodes/barcodes.md

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+---
+title: Barcodes
+---
+
+## Barcode Support
+
+InvenTree has native support for barcodes, which provides powerful functionality "out of the box", and can be easily extended:
+
+- Barcodes can be scanned [via the API](../api/api.md)
+- The web interface supports barcode scanning
+- Barcodes integrate natively [with the mobile app](../app/barcode.md)
+- Custom barcodes can be assigned to items
+- Barcodes can be embedded in [labels or reports](../report/barcodes.md)
+- Barcode functionality can be [extended via plugins](../extend/plugins/barcode.md)
+
+### Barcode Data Types
+
+Barcodes can be linked with the following data model types:
+
+- [Part](../part/part.md#part)
+- [Stock Item](../stock/stock.md#stock-item)
+- [Stock Location](../stock/stock.md#stock-location)
+- [Supplier Part](../buy/supplier.md#supplier-parts)
+
+## Web Integration
+
+Barcode scanning can be enabled within the web interface. Barcode scanning in the web interface supports scanning via:
+
+- Keyboard style scanners (e.g. USB connected)
+- Webcam (image processing)
+
+### Configuration
+
+Barcode scanning may need to be enabled for the web interface:
+
+{% with id="barcode_config", url="barcode/barcode_settings.png", description="Barcode settings" %}
+{% include 'img.html' %}
+{% endwith %}
+
+### Scanning
+
+When enabled, select the barcode icon in the top-right of the menu bar to scan a barcode. If the barcode is recognized by the system, the web browser will automatically navigate to the correct item:
+
+{% with id="barcode_scan", url="barcode/barcode_scan.png", description="Barcode scan" %}
+{% include 'img.html' %}
+{% endwith %}
+
+#### No Match Found
+
+If no match is found for the scanned barcode, the following error message is displayed:
+
+{% with id="barcode_no_match", url="barcode/barcode_no_match.png", description="No match for barcode" %}
+{% include 'img.html' %}
+{% endwith %}
+
+## App Integration
+
+Barcode scanning is a key feature of the [companion mobile app](../app/barcode.md).

docs/barcodes/custom.md

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+---
+title: Custom Barcodes
+---
+
+## Custom Barcode Functionality
+
+With the provision of [internal](./internal.md) and [external](./external.md) barcode support, a lot of potential use-cases are already supported directly by InvenTree.
+
+However, if further customization is required, or a bespoke barcode workflow which is not supported already, then this can easily be implemented using the [plugin system](../extend/plugins/barcode.md).
+
+A custom barcode plugin can be used to (for example) perform a particular action when a barcode is scanned.
+
+### Scanning a Barcode
+
+To scan (process) a barcode, the barcode data is sent via a `POST` request to the `/api/barcode/` API endpoint.
+
+### Barcode Scanning Priority
+
+When a barcode is scanned (sent to the `/barcode/scan/` endpoint), each available "plugin" is checked to see if it returns a valid result for the provided barcode data. The first plugin to return a result prevents any further plugins from being checked.
+
+The barcode is tested as follows, in decreasing order of priority:
+
+- [Internal Barcode Plugin](./internal.md)
+- [External Barcode Plugin](./external.md)
+- [Custom Barcode Plugins](../extend/plugins/barcode.md)
+
+!!! tip "Plugin Loading Order"
+    The first custom plugin to return a result "wins". As the loading order of custom plugins is not defined (or configurable), take special care if you are running multiple plugins which support barcode actions.

docs/barcodes/external.md

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+---
+title: External Barcodes
+---
+
+## External Barcodes
+
+In addition to defining an [internal barcode format](./internal.md), models which have associated barcode information also allow arbitrary external (third party) barcodes to be assigned or "linked" to items in the database.
+
+For example, you have just purchased a reel of capacitors from a supplier, which comes provided with a sufficiently unique barcode or qr-code. Instead of printing an *internal* barcode, the existing barcode can be scanned and *linked* to the specific reel (which is a [Stock Item](../stock/stock.md#stock-item)).
+
+Linking to external barcodes allows an alternative barcode workflow, which may be especially useful when dealing with in-feed components which are received from external suppliers.
+
+!!! tip "Dealer's Choice"
+    The use of external barcodes is entirely up to the user, if it is deemed to be convenient.
+
+## Linking Barcodes
+
+### Via the API
+
+Facility for barcode linking (and un-linking) is provided via the [API](../api/api.md).
+
+- The `/api/barcode/link/` API endpoint is used to link a barcode with an existing database item
+- The `/api/barcode/unlink/` API endpoint is used to unlink a barcode from an existing database item
+
+### Via the Web Interface
+
+To link an arbitrary barcode, select the *Link Barcode* action as shown below:
+
+{% with id="barcode_link_1", url="barcode/barcode_link_1.png", description="Link barcode" %}
+{% include 'img.html' %}
+{% endwith %}
+
+{% with id="barcode_link_2", url="barcode/barcode_link_2.png", description="Link barcode" %}
+{% include 'img.html' %}
+{% endwith %}
+
+If an item already has a linked barcode, it can be un-linked by selecting the *Unlink Barcode* action:
+
+{% with id="barcode_unlink", url="barcode/barcode_unlink.png", description="Unlink barcode" %}
+{% include 'img.html' %}
+{% endwith %}
+
+### Via the App
+
+External barcodes can be linked to (or unlinked from) database items via the [mobile app](../app/barcode.md)

docs/barcodes/internal.md

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+---
+title: Internal Barcodes
+---
+
+## Internal Barcodes
+
+InvenTree defines an internal format for generating barcodes for various items. This format uses a simple JSON-style string to uniquely identify an item in the database.
+
+Some simple examples of this format are shown below:
+
+| Model Type | Example Barcode |
+| --- | --- |
+| Part | `{% raw %}{"part": 10}{% endraw %}` |
+| Stock Item | `{% raw %}{"stockitem": 123}{% endraw %}` |
+| Supplier Part | `{% raw %}{"supplierpart": 99}{% endraw %}` |
+
+The numerical ID value used is the *Primary Key* (PK) of the particular object in the database.
+
+## Report Integration
+
+This barcode format can be used to generate 1D or 2D barcodes (e.g. for [labels and reports](../report/barcodes.md))
+
+To access the raw barcode information string within a template, use the `.barcode` attribute, and pass it into a barcode generation method.
+
+### Example: QR Code
+
+For example, to render a QR-Code image for a part instance:
+
+```html
+{% raw %}
+<img src='{% qrcode part.barcode %}'>
+{% endraw %}
+```
+
+!!! info "Barcode Formatting"
+    Refer to the [report documentation](../report/barcodes.md) for further information on formatting barcode data