[DOCS] Registration for custom linkvalidator linktypes

Introduce development and API section, so they can be referenced.

Resolves: #97140
Releases: main
Change-Id: I28051e09c7800c2653b288460bb618c2bcb987f7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73857


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>