Add a tutorial to package Godot projects as Flatpaks

Now that there is a [Godot BaseApp](https://github.com/flathub/org.godotengine.godot.BaseApp) available on Flathub, we should write a tutorial to package a Godot project as a Flatpak. It should strive to be as short as possible and cover only the essentials, such as:

- Creating the required `.desktop` and AppStream metadata files. We can add templates to the documentation page itself.
- Creating a Flatpak manifest (based on a template) and editing it to suit the project.
- Setting up sandbox permissions depending on the project's needs.

For more advanced use cases, it should link to the [Flatpak documentation](https://docs.flatpak.org/en/latest/).