[Compiling for macOS on Linux] Adding instructions on how to install required MoltenVK

**Your Godot version:** 4.2

**Issue description:** Adding information about how to cross-compile for macOS from Linux with Vulkan support.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/contributing/development/compiling/compiling_for_macos.html#cross-compiling-for-macos-from-linux

I recently tried to cross-compile export templates for macOS, using my Linux server. I ran in to an error saying the Vulkan SDK couldn't be found. There are no instructions on how to install it (for MoltenVK). It seems like this part was copied from 3.x because no Vulkan support was introduced back then. I suggest an addition because the build process is not working with this setup. (also I don't know how to do it, so I can't contribute)