Rework API documentation

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

doc/API-GNU-Social.md

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+# GNU Social API
+
+* [Home](help)
+  * [Using the APIs](help/api)
+
+## Overview
+
+Friendica provides the following endpoints defined in [the official GNU Social Twitter-like API reference](https://gnusocial.net/doc/twitterapi).
+
+Authentication is the same as described in [Using the APIs](help/api#Authentication).
+
+## Entities
+
+These endpoints use the [Friendica API entities](help/API-Entities).
+
+## Implemented endpoints
+
+- GET api/account/rate_limit_status
+- POST api/account/update_profile_image
+- GET api/account/verify_credentials
+
+- GET api/direct_messages
+- POST/DELETE api/direct_messages/destroy
+- POST api/direct_messages/new
+- GET api/direct_messages/sent
+- GET api/favorites
+- POST api/favorites/create/:id
+- POST api/favorites/destroy/:id
+- GET api/followers/ids
+- POST api/friendships/destroy
+- GET api/friends/ids
+- GET/POST api/help/test
+- POST api/oauth/access_token
+- POST api/oauth/request_token
+- GET api/search
+- GET api/statuses/show/:id
+- POST api/statuses/destroy/:id
+- GET api/statuses/followers
+- GET api/statuses/friends
+- GET api/statuses/friends_timeline
+- GET api/statuses/friends_timeline/:username
+- GET api/statuses/home_timeline
+- GET api/statuses/mentions
+- GET api/statuses/replies
+- GET api/statuses/replies/:username
+- POST api/statuses/retweet/:id
+- GET api/statuses/public_timeline
+- POST api/statuses/update
+- GET api/statuses/user_timeline
+- GET api/users/show
+
+## Non-implemented endpoints
+
+- statuses/retweeted_to_me
+- statuses/retweeted_by_me
+- statuses/retweets_of_me
+- friendships/create
+- friendships/exists
+- friendships/show
+- account/end_session
+- account/update_delivery_device
+- account/update_profile_background_image
+- notifications/follow
+- notifications/leave
+- blocks/create
+- blocks/destroy
+- blocks/exists
+- blocks/blocking
+- oauth/authorize
+- statusnet/groups/timeline
+- statusnet/groups/show
+- statusnet/groups/create
+- statusnet/groups/join
+- statusnet/groups/leave
+- statusnet/groups/list
+- statusnet/groups/list_all
+- statusnet/groups/membership
+- statusnet/groups/is_member
+- statusnet/tags/timeline
+- statusnet/media/upload
+- statusnet/config