Add developer docs for application registry

docs/development/application-registry.md

@@ -0,0 +1,55 @@
+# Application Registry
+
+The registry is an in-memory data structure which houses various miscellaneous application-wide parameters, such as installed plugins. It is not exposed to the user and is not intended to be modified by any code outside of NetBox core.
+
+The registry behaves essentially like a Python dictionary, with the notable exception that once a store (key) has been declared, it cannot be deleted or overwritten. The value of a store can, however, me modified; e.g. by appending a value to a list. Store values generally do not change once the application has been initialized.
+
+## Stores
+
+### `model_features`
+
+A dictionary of particular features (e.g. custom fields) mapped to the NetBox models which support them, arranged by app. For example:
+
+```python
+{
+    'custom_fields': {
+        'circuits': ['provider', 'circuit'],
+        'dcim': ['site', 'rack', 'devicetype', ...],
+        ...
+    },
+    'webhooks': {
+        ...
+    },
+    ...
+}
+```
+
+### `plugin_menu_items`
+
+Navigation menu items provided by NetBox plugins. Each plugin is registered as a key with the list of menu items it provides. An example:
+
+```python
+{
+    'Plugin A': (
+        <MenuItem>, <MenuItem>, <MenuItem>,
+    ),
+    'Plugin B': (
+        <MenuItem>, <MenuItem>, <MenuItem>,
+    ),
+}
+```
+
+### `plugin_template_extensions`
+
+Plugin content that gets embedded into core NetBox templates. The store comprises NetBox models registered as dictionary keys, each pointing to a list of applicable template extension classes that exist. An example:
+
+```python
+{
+    'dcim.site': [
+        <TemplateExtension>, <TemplateExtension>, <TemplateExtension>,
+    ],
+    'dcim.rack': [
+        <TemplateExtension>, <TemplateExtension>,
+    ],
+}
+```

mkdocs.yml

@@ -71,6 +71,7 @@ nav:
         - Style Guide: 'development/style-guide.md'
         - Utility Views: 'development/utility-views.md'
         - Extending Models: 'development/extending-models.md'
+        - Application Registry: 'development/application-registry.md'
         - Release Checklist: 'development/release-checklist.md'
         - Squashing Migrations: 'development/squashing-migrations.md'
     - Release Notes: