Update plugin docs

- Create separate files for each mixin
- Remove old "report" plugin docs

docs/extend/plugins.md

@@ -16,9 +16,59 @@ Plugins are discovered and loaded when the server is started.
 !!! info "Enable Plugin Support"
     To enable custom plugins, plugin support must be activated in the [server configuration](../start/config.md).
 
-Multiple plugins are supported:
+### Plugin Base Class
 
-- [Reporting plugins](./plugins/report.md)
-- [Barcode plugins](./plugins/barcode.md)
-- [Action plugins](./plugins/action.md)
-- [Integration plugins](./plugins/integration.md)
+Custom plugins must inherit from the [IntegrationPluginBase class](https://github.com/inventree/InvenTree/blob/master/InvenTree/plugin/integration.py). Any plugins installed via the methods outlined above will be "discovered" when the InvenTree server launches.
+
+### Plugin Options
+
+Some metadata options can be defined as constants in the plugins class
+
+``` python
+PLUGIN_SLUG = None  # Used in URLs, setting-names etc. when a unique slug as a reference is needed -> the plugin name is used if not set
+PLUGIN_TITLE = None  # A nice human friendly name for the plugin -> used in titles, as plugin name etc.
+
+AUTHOR = None  # Author of the plugin, git commit information is used if not present
+PUBLISH_DATE = None  # Publishing date of the plugin, git commit information is used if not present
+VERSION = None  # Version of the plugin
+WEBSITE = None  # Website for the plugin, developer etc. -> is shown in plugin overview if set
+```
+
+Refer to the [sample plugins](https://github.com/inventree/InvenTree/tree/master/InvenTree/plugin/samples) for further examples.
+
+### Plugin Config
+
+A *PluginConfig* database entry will be created for each plugin "discovered" when the server launches. This configuration entry is used to determine if a particular plugin is enabled.
+
+The configuration entries must be enabled via the [InvenTree admin interface](../settings/admin.md).
+
+!!! warning "Disabled by Default"
+    Newly discovered plugins are disabled by default, and must be manually enabled by an admin user
+
+### Plugin Mixins
+
+Common use cases are covered by pre-supplied modules in the form of mixins (similar to how [django](https://docs.djangoproject.com/en/stable/topics/class-based-views/mixins/) does it). Each mixin enables the integration into a specific area of InvenTree. Sometimes it also enhances the plugin with helper functions to supply often used functions out-of-the-box.
+
+Supported mixin classes are:
+
+- [ActionMixin](./plugins/action.md)
+- [APICallMixin](./plugins/api.md)
+- [AppMixin](./plugins/app.md)
+- [BarcodeMixin](./plugins/barcode.md)
+- [EventMixin](./plugins/event.md)
+- [NavigationMixin](./plugins/navigation.md)
+- [ScheduleMixin](./plugins/schedule.md)
+- [SettingsMixin](./plugins/settings.md)
+- [UrlsMixin](./plugins/urls.md)
+
+## Installing a Plugin
+
+Plugins can either be loaded from paths in the InvenTree install directory or as a plugin installed via pip. We recommend installation via pip as this enables hassle-free upgrades.
+
+For development new plugins can be placed ina a subdirectory in `src/InvenTree/plugins`. Built-In plugins ship in `src/InvenTree/plugin/builtin`. To achive full unit-testing for all mixins there are some sample implementations in `src/InvenTree/plugin/samples`. These are not loaded in production mode.
+
+### Plugin Installation File
+
+Plugins installation can be simplified by providing a list of plugins in a plugin configuration file. This file (by default, 'plugins.txt' in the same directory as the server configuration file) contains a list of required plugin packages.
+
+Plugins can be installed from this file by simply running the command `invoke plugins`.