Docs: Added API docs for using Safe Settings

[primetheus]

This provides a more clear set of documentation that is based on GitHub's API docs. It makes it more clear how one can structure their YAML files to function properly

cc: @decyjphr

[ElioDiNino]

Hey @primetheus, this is really great and I hope it gets merged in. I notice that you are missing docs for milestones and rulesets though. Do you plan on adding this?

[decyjphr]

@primetheus thank you! I agree this is a great addition and gives us a template for adding content for other settings.