Documentation for callback API and extensions

[S-S-X]

Improve documentation for beerchat.

• [ ] Add documentation for hooks.lua API.
• [ ] Add doucmentation for included extensions like jail.lua.

[S-S-X]

Also doc/manual.md content does not really make sense anymore, could possibly just remove that file completely and rewrite API docs in some doc/API.md and then add basic capability/command/configuration manual directly to README.md.

Most of it is anyway documented well enough with /help and only some special stuff really needs documentation in repo: channel usage with #, whispering with $ and private messages with @.

Extensions could have separate file for documentation? Currently there's:

• server-announce where additional chat command doc might be useful
• password check behavior
• cleaner behavior
• colorize behavior
• jail configuration

I think most other stuff is documented well enough with /help whateverbeerchatcommand

[S-S-X]

Additionally missing docs for /cc channel update mechanism. Like a lot of other stuff this is in /help but not in docs.