[DOCS] Streamline new module registration docs

Fix some typos and visual glitches.
Additionally, correct the headings in Index.rst.

Resolves: #96827
Related: #96733
Releases: main
Change-Id: Ic32b6ad7dc677547853e0b681eff9d6cf3148467
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73420


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>