Documentation for Application Credentials platform initial features

docs/core/platform/application_credentials.md

@@ -0,0 +1,54 @@
+---
+title: "Application Credentials"
+---
+
+Integrations support [Configuration via OAuth2](https://developers.home-assistant.io/docs/config_entries_config_flow_handler#configuration-via-oauth2) and the preferred approach is to use the Home Assistant Cloud Account Linking service. Integrations may also allow users to provide their own OAuth client credentials by adding a `application_credentials.py` and implementing the right functions.
+
+:::note
+Application Credentials is under active development and integrations should still prefer using `LocalOAuth2Implementation`.
+:::
+
+## Adding support
+
+Integrations support application credentials with a file in the integration folder called `application_credentials.py` and implement the following:
+
+```python
+from homeassistant.core import HomeAssistant
+from homeassistant.components.application_credentials import AuthorizationServer
+
+
+async def async_get_authorization_server(
+    self, hass: HomeAssistant
+) -> AuthorizationServer:
+    """Return authorization server."""
+    return AuthorizationServer(
+        authorize_url="https://example.com/auth",
+        token_url="https://example.com/oauth2/v4/token"
+    )
+```
+
+See below for details on backwards compatibility with YAML credentials.
+
+## AuthorizationServer
+
+An `AuthorizationServer` represents the [OAuth2 Authorization server](https://datatracker.ietf.org/doc/html/rfc6749) used for an integration.
+
+| Name          | Type |                                                                                                    | Description |
+| ------------- | ---- | -------------------------------------------------------------------------------------------------- | ----------- |
+| authorize_url | str  | **Required** | The OAuth authorize URL that the user is redirected to during the configuration flow. |
+| token_url     | str  | **Required** | The URL used for obtaining an access token.                                           |
+
+## Import YAML credentials
+
+Credentials may be imported by integrations that used to accept YAML credentials using the import API `async_import_client_credential` provided by the application credentials integration.
+
+The `auth_domain` is the domain for the auth implementation in the config entry, which is typically the domain specified in  an existing `LocalOAuth2Implementation`.
+
+## ClientCredential
+
+A `ClientCredential` represents a client credential provided by the user.
+
+| Name          | Type |                                                                           | Description |
+| ------------- | ---- | ------------------------------------------------------------------------- | ----------- |
+| client_id     | str  | **Required** | The OAuth Client ID provided by the user.     |
+| client_secret | str  | **Required** | The OAuth Client Secret provided by the user. |