[DOCS] Document known issues with ext:linkvalidator

Linkvalidator extension has a couple of known issues,
especially with external link checking.

Those issues are now documented, so integrators can
acknowledge them and implement counter-measures.

Resolves: #101597
Releases: main, 12.4, 11.5
Change-Id: I00dc01243426fc56c21a4bd11815c3009e3dd3ad
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80364


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