Create a template repository with a premade SCons setup

[GitHub allows repositories to be marked as "template repositories".](https://github.blog/2019-06-06-generate-new-repositories-with-repository-templates/) Perhaps we could make use of this to create a repository that's ready to be cloned and built.

The template could include a class that just prints `Hello world!` or something along those lines, then people could extend it with additional files as needed. Maybe it should also contain `.gdns`/`.gdnlib` files? I'm not sure about that part.

I've seen a handful of third-party efforts on this, but it would be nice to have something official. This would likely be a better place to maintain such examples than the [documentation pages](https://github.com/godotengine/godot-docs/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+gdnative).