Improved headless documentation (including build instructions)

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.