Add detailed instructions for git and GitHub auth + signing

[ardigan6]

This PR adds step by step notes to make things a bit more point and click.

The reasons this initially didn't work for me were scattered across many issues in the repo, and ultimately had to review the source code to get this working.

Also ran across several bugs on 13.1, with workarounds listed under Troubleshooting.

[ardigan6]

Issues documented in: https://github.com/maxgoedjen/secretive/issues/440