Document common git how tos for working with branches-of-branches

### What problem does your feature request solve?

When working with branches-of-branches it be confusing how to stay up-to-date with your base branch, and we don't have any documentation for it.

### Describe the solution you'd like

Add scenario based git usage guide for working with branches of branch and updating main. Include diagrams.

### Describe alternatives you've considered

No documentation file or entry point is named in the issue, so first locate the repository's existing Git guides and diagram conventions. Add scenario-based guidance for keeping branches-of-branches up to date with their base branch and updating main, with diagrams; done means these workflows are documented clearly.

Written by the indexing model from the issue text.