docs: How to rebase a branch before a PR

### Overview

We need documentation for how to rebase a development branch in preparation for opening a PR so that developers can avoid conflicts in their PRs. This happens often enough that we should create this guide even before the MkDocs migration.

### Action Items

- [x] documentation for rebasing a branch onto the latest `main`
- [ ] instructions for resolving merge conflicts
- [ ] instructions for resolving migration file conflicts
- [ ] go to #189, check off the item relating to this issue, and close #189

### Resources/Instructions

- compare rebase with merge, why we want to choose rebase over merge
- steps to rebase
- make sure the code works well in the branch
- optional interactive rebase to simplify commit history
- fetch hackforla/main
- git rebase hackforla/main
- resolve merge conflicts
- resolve migration file conflicts