📘 DOC: Add instructions about “git rebase” for contributing about docs

[MarcusOtter]

Guilty 🙋‍♂️ (though I do rebase with my git pulls 😉)
(for reference: withastro/astro#927 and withastro/astro#928)

Use cases:

• Merge conflict on my pull request, and my fork is a few days behind astro/main. What do? (#928)
• Feature branch is behind by a few commits of the main branch. What do?
  Usually I would merge the main branch into the feature branch (perhaps with --ff-only)

Seems like merging ends up with very messy pull requests (#927) or a messy commit history? So I guess an overview on the git workflow of this repository makes sense to have in the contribution guidelines. However, fully fledged instructions about specific git features sounds out of scope, we should probably link to the official git docs for that (and maybe some easy-to-understand examples using said features)

[sarah11918]

I'm going to close this issue, since @delucis is currently putting together some contributing instrutions, I think including mention of the GitHub app "Pull" that people can install to automatically fetch upstream reguarly. Chris, let's take this into consideration when creating a workflow for contributors! If y'all don't like what is produced, we can revisit focused on the new thing. :)