docs(contrib): add notes on dealing with merge conflicts

[agilgur5]

Addresses some of my comments in #13631 (comment) and adds some things I've noted on Slack before too

Motivation

Add two small best practices for dealing with merge conflicts that I've been using

• can add more to this section later as we iron them out

Plus clarify a contradictory piece that may have confused Alan

Modifications

• split out a "Merge Conflicts" section for this
  • fix up some formatting in the existing sentence to match the docs style guide
  • add what to do if you're not sure how to fix it: don't backport it is the safer option
  • add instruction to add a note in the commit if you did fix a merge conflict
    • add prior examples of this as well

Other modifications

• remove chore(deps), build, and ci from the "list of commits"

  • matching the "Ignore: " below from docs: Improve release instructions #11382
  • these also tend to cause the most merge conflicts, directly or indirectly

• add docs to this list since we now have versioned documentation Versioned documentation #11390

  • i.e. docs-only improvements unrelated to a feature should also be backported when possible, as previously discussed and as I've done many times now and Alan's done a few times now too

• add note about pinning release announcements in Slack

  • use the specific terms that the Slack UI uses: "Pin" and "Un-pin"
  • this is something new I've been doing recently

Verification

make docs passes