[TASK] Changelog docs: Update Howto.rst

Add more .rst file location information to the Changelog
"Documenting Changes" Howto.rst.
Also add missing Index.rst entries for 8.7.x/ files.

Change-Id: Ie68e5eb59f080537050717106bf549dc6d4a0ef3
Resolves: #83371
Releases: master, 8.7, 7.6
Reviewed-on: https://review.typo3.org/55139


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>