[DOCS] Restructure API Reference

The API Reference of the form framework resided inside the docs for
integrators. This patch creates a new main chapter "For Developers/
API Reference" and moves all the related content there. In addition,
the content has been splitted in three documents.

Resolves: #97039
Releases: main, 11.5
Change-Id: Ie626f3ebdd137dfb7aef308b1bf7568dc17de985
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73710


Tested-by: Colin Findeisen <colin.findeisen@tritum.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Colin Findeisen <colin.findeisen@tritum.de>
Reviewed-by: waldhacker <hello@waldhacker.dev>