Unify documentation efforts

[mark-wiemer]

Now that docs.luanti.org is "working", we should re-evaluate how documentation is handled across Luanti. We currently have documentation in these places:

• luanti.org
• docs.luanti.org
• api.luanti.org (which is just a nice version of engine/docs/lua_api.md)
• various documentation files in the engine
• other places?

Ideally, there is clarity on how and when to update each of these locations.

Starting this issue to discuss alignment work here and get team feedback on next steps