[TASK] Changelog docs: Review and add tags

* Add tags of all v7 Changelog files
* Review existing tags of all v8 & v9 Changelog files
* Minor .rst reformattings and fixes

Change-Id: I4761a1d8028feca5de30a3f860fd5f01bbdd42d1
Resolves: #83402
Releases: master, 8.7, 7.6
Reviewed-on: https://review.typo3.org/55182


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>