[DOCS] Add regex example to target column referencing capturing groups

This patch adds necessary hints about regex capturing groups
to the documentation of the field target of EXT:redirects.

Both columns (source_path and _target) referencing the
regular expression functionality from now on.

Resolves: #103272
Releases: main, 12.4, 11.5
Change-Id: I764184c306a2fb7176da17e7d2e5f1e6ffae467b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83274


Reviewed-by: Lina Wolf <112@linawolf.de>
Tested-by: Lina Wolf <112@linawolf.de>
Tested-by: core-ci <typo3@b13.com>