[3.12] Update the documentation howto index page and group docs into 3 logical sections (GH-119366) (#127811)

(cherry picked from commit a26d27e7ee512cd822b7a7ba075171152779ffdd)

Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>