Rework API documentation

- Split API pages by target software
- Added API entities list

doc/API-Mastodon.md

@@ -0,0 +1,24 @@
+# Mastodon API
+
+* [Home](help)
+  * [Using the APIs](help/api)
+
+## Overview
+
+Friendica provides the following endpoints defined in [the official Mastodon API reference](https://docs.joinmastodon.org/api/).
+
+Authentication is the same as described in [Using the APIs](help/api#Authentication).
+
+## Entities
+
+These endpoints use the [Mastodon API entities](https://docs.joinmastodon.org/api/entities/).
+
+## Implemented endpoints
+
+- [GET /api/v1/follow_requests](https://docs.joinmastodon.org/api/rest/follow-requests/#get-api-v1-follow-requests)
+
+## Non-implemented endpoints
+
+- [POST /api/v1/follow_requests/:id/authorize](https://docs.joinmastodon.org/api/rest/follow-requests/#post-api-v1-follow-requests-id-authorize)
+- [POST /api/v1/follow_requests/:id/reject](https://docs.joinmastodon.org/api/rest/follow-requests/#post-api-v1-follow-requests-id-reject)
+