Add and update API for standalone signaling server from wiki.
Add and update API for standalone signaling server from wiki.

Better to keep that in the repository similar to the other API documentation.

Signed-off-by: Joachim Bauch <f7be0a2a0fb4eba859c8fa4289260059dc564a0d@struktur.de>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed