Document the Godot release process

In the interest of transparency, we should document how a Godot release is made:

- Which build system is used (https://github.com/godotengine/build-containers).
- Documentation on using the build containers themselves is likely better put in a README file in that repository.
- How Windows binaries are signed (and soon macOS).
- Which other steps are taken before a release is announced in general.

Examples:

- [Minetest](https://dev.minetest.net/Releasing_Minetest)
- This one is pretty detailed. It might be one of the best examples I know.
- [Visual Studio Code](https://github.com/microsoft/vscode/wiki/Release-Process)

This page should likely be added in the Development section.