Improved headless documentation (including build instructions)

[austinried]

Not exactly sure where this should go so feel free to correct me if I missed here.

I would love to see more official documentation on how to use the headless version of Godot, primarily because I feel like it is the ideal way to export projects (in a CI/CD system). Currently there are only two hits for headless in the official docs, and they are just brief mentions. Topics that would be great to cover:

• Exporting projects with headless (CLI options, examples, etc.)
• Common exporting "gotchas" (main example I can think of here is that when exporting with headless for mac from windows/linux, if you use a .zip extension via CLI you only get the pck file, whereas through the editor if you use a .zip extension you get the full app bundle zip)
• Build instructions

I'm not sure if that last one actually requires work on the build system or not, but I have not yet found or figured out how to build headless on my own, and would love to be able to do that. Especially since for my workflow I keep scripts around for building the engine/editor, and including headless in that since I use it too would be great.

[Calinou]

I suppose we should have a dedicated page on setting up various CI services with Godot for automated exporting.

[Calinou]

There's now documentation for exporting to dedicated servers: https://docs.godotengine.org/en/stable/tutorials/export/exporting_for_dedicated_servers.html

There's still no page about exporting using CI platforms, but there is no official template for this yet. godot-ci is technically third-party, and I think we should only endorse official resources for this considering exporting from CI platforms can have security implications.