TYPO3 Core

Previously, it used to be possible to replace and override an existing linktype without XCLASS.

With the change https://review.typo3.org/c/Packages/TYPO3.CMS/+/73568, the linktypes are automatically registered so it is not possible to deactivate or replace one of them directly. (It is still possible with XCLASS).

- check what is current best practice for doing this
- document it

Currently we have an example for creating a new custom type: https://docs.typo3.org/c/typo3/cms-linkvalidator/main/en-us/Development/LinkTypeImplementation.html

AFAIK the identifier of the linktype must correspond to the link type returned by the link parsers, so we can't just use a different identifier.

A different identifier must be used for the new custom class.

Since we recommend to "override the ExternalLinktype class" as one possibility in Known problems, this should also be documented.