[TASK] Improve readability of 8.x Changelog

[TASK] Improve readability of 8.x Changelog

Only breaking changes and important changes were adjusted for now.

* Fix rst syntax issues, like missing blank line before list.
* Add roles for highlighting of inline files and code.
* Add link to mentioned issues.
* Prefix PHP Globals with $GLOBALS[] to keep things as they are in real
  world and to keep it the same between all files.
* Use single formats, e.g. always '*' for lists. This will make
  refactorings and other stuff easier.

Resolves: #85771
Releases: master, 8.7
Change-Id: I47f7db732ab1da751c6f84469152d7209e065c19
Reviewed-on: https://review.typo3.org/57841
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>