Do manual squash cleanup for docs (#6646)

Do manual squash cleanup for docs (#6646)

This PR abandons the attempt to do the cleanup in a single rebase command, and instead squashes the old commits manually. The current solution does not handle conflicts properly. The new approach (which seems to be more robust) proceeds as follows (to squash $first..$last):

- checkout $last
- soft reset to $first (checkout $first, but keep changes in working copy)
- commit to squashed commit
- cherry-pick $last..HEAD from main branch