Create and document our commit workflow

At the moment, each project has been maintained slightly differently, but with the branch changes we're taking the opportunity to document and consolidate our workflows on each of these projects. They don't all need to be handled the same, but we should definitely document the different workflows and point out projects that have exceptions. This workflow should include the following:

• which repositories to push to and where our mirrors are pointing
• do we introduce merge commits or do we rebase branches before merging?
• do we use the gitlab interface or merge things locally?
• how many reviews do we need and who maintains/has access to which repository?
• we had some discussion over on #7 about signing commits
• which projects have releases and what is the release workflow?

This is generally a good idea, and something we should work into our workflow. Let's use this ticket to document a proposal for different workflows. Again, some repositories for our team are maintained by people outside TPI so the focus should be on documentation and best practices, not necessarily in making everything the same.