[Docs][Github] explain how to rectify gh pr merge failure (#66223)

I recently went to merge a PR that had a merge conflict:

    $ gh pr merge --squash --delete-branch
X Pull request #66003 is not mergeable: the merge commit cannot be
cleanly created.
To have the pull request merged after all the requirements have been
met, add the `--auto` flag.
    Run the following to resolve the merge conflicts locally:
gh pr checkout 66003 && git fetch origin main && git merge origin/main

This is how I resolved it; we should recommend this explicitly for
fellow contributors.