Documentation integration (#4653)

* Add documentation under docs/ directory

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docs/docs/api/metadata.md

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+---
+title: Model Metadata
+---
+
+## Model Metadata
+
+The API is *self describing* in that it provides metadata about the various fields available at any given endpoint. External applications (such as the [python interface](../api/python/python.md)) can introspect the API to determine information about the model fields.
+
+!!! tip "API Forms"
+    The various forms implemented in the InvenTree web interface make heavy use of this metadata feature
+
+### Requesting Metadata
+
+To request metadata about a particular API endpoint, simply perform an `OPTIONS` method request against the API URL.
+
+For example, to view the metadata available for creating a new [Part Category](../part/part.md#part-category), an `OPTIONS` request to `/api/part/category/` yields:
+
+{% with id="api_cat_options", url="api/api_category_options.png", description="Part category options" %}
+{% include 'img.html' %}
+{% endwith %}
+
+You can see here a detailed list of the various fields which are available for this API endpoint.
+
+## Metadata Information
+
+The `OPTIONS` endpoint provides the following information:
+
+| Entry | Description |
+| --- | --- |
+| name | The human-readable name of the API endpoint |
+| description | Descriptive detail for the endpoint, extracted from the python docstring |
+| actions | Contains the available HTTP actions and field information (see below) |
+
+Specific details are provided on the available attributes of each field:
+
+{% with id="api_fields", url="api/api_metadata_fields.png", description="Metadata fields" %}
+{% include 'img.html' %}
+{% endwith %}
+
+### Field Types
+
+Supported field types are:
+
+| Field Type | Description |
+| --- | --- |
+| string | Text data |
+| boolean | true / false value |
+| integer | Integer numbers |
+| float | Floating point numbers |
+| related field | Primary key value for a foreign-key relationship in the database |
+
+### Field Attributes
+
+Each named field provides information on available attributes:
+
+| Attribute | Description |
+| --- | --- |
+| type | Defines the [field type](#field-types) |
+| default | The default value for this field. Will be assumed if no value is supplied |
+| required | Boolean value, whether this field must be supplied |
+| read_only | Boolean value, whether this field is writeable |
+| label | Human readable descriptive label for this field. |
+| help_text | Long form descriptor for this field. |
+| min_value | Minimum allowed value (for numeric fields) |
+| max_value | Maximum allowed value (for numeric fields) |
+| max_length | Maximum allowed length (for text fields) |
+| model | Name of the database model, if this field represents a foreign-key relationship |
+| api_url | API url for the related model, if this field represents a foreign-key relationship |
+| filters | API filters for the field, if this field represents a foreign-key relationship |
+
+!!! tip "Field Name"
+    The field name is the *key* used to define the field itself
+
+!!! info "Available Attributes"
+    Some attributes may not be made available for a particular field
+
+
+
+## Translation
+
+Field *label* and *help text* values are localized using the [community contributed translations](https://crowdin.com/project/inventree). The required locale information is determined from the API request itself, meaning that the translated values are provided automatically.
+
+For example, the same forms (in the web interface) are served via identical API requests, with the locale information determined "on the fly":
+
+{% with id="api_english", url="api/api_english.png", description="API forms (english)" %}
+{% include 'img.html' %}
+{% endwith %}
+
+{% with id="api_german", url="api/api_german.png", description="API forms (german)" %}
+{% include 'img.html' %}
+{% endwith %}