[DOCS] Add hint about relative targets in redirects

This patch extends the documentation of typo3/cms-redirects by
an example of redirecting to an relative target while using
regular-expressions. This adds more clarity for TYPO3 integrators
and administrators.

Resolves: #99639
Releases: main, 11.5
Change-Id: I6f6e24b5cf0a378d0efe0c7266c9abf249559e7d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77438


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>