docs(api): 📝 Document all API changes

docs/api/mastodon.md

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+# Mastodon API Extensions
+
+Extra attributes have been added to some Mastodon API routes. Changes are documented in this document.
+
+## `/api/v1/instance`
+
+Three extra attributes have been added to the `/api/v1/instance` endpoint:
+
+```ts
+{
+    // ...
+    banner: string | null;
+    lysand_version: string;
+    sso: {
+        forced: boolean;
+        providers: {
+            id: string;
+            name: string;
+            icon?: string;
+        }[];
+    }
+}
+```
+
+### `banner`
+
+The URL of the instance's banner image. `null` if there is no banner set.
+
+### `lysand_version`
+
+The version of the Lysand instance.
+
+The normal `version` field is always set to `"4.3.0+glitch"` or similar to not confuse clients that expect a Mastodon instance.
+
+### `sso`
+
+Single Sign-On (SSO) settings for the instance. This object contains two fields:
+
+- `forced`: If this is enabled, normal identifier/password login is disabled and login must be done through SSO.
+- `providers`: An array of external OpenID Connect providers that users can link their accounts to. Each provider object contains the following fields:
+  - `id`: The issuer ID of the OpenID Connect provider.
+  - `name`: The name of the provider.
+  - `icon`: The URL of the provider's icon. Optional.
+
+## `/api/v2/instance`
+
+Contains the same extensions as `/api/v1/instance`, except `banner` which uses the normal Mastodon API attribute.