[DOCS] Improve changelog for v12 release

- Add cross references for changelog files
- Remove additional empty lines
- Rename core -> Core
- Rename composer -> Composer
- Fix grammar: extensions' -> extension's
- Indent lists with 4 spaces (including the asterisk character)
- Indent code-blocks with 4 spaces (including the two dots)
- Simplify code examples
- Fix some typos and simplify sentences along the way

Releases: main
Resolves: #98498
Change-Id: I70f0b68ba9bcf69f6ac70d8aef95f70a0cb10923
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75938


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Chris Müller <typo3@krue.ml>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>