[TASK] Deprecate ext_emconf.php key "docPath"

Resolves: #53307
Releases: 6.2
Documentation: #53308
Change-Id: Ib283f5cb6620e2e359899d0523ec4afe1dd0d15c
Reviewed-on: https://review.typo3.org/25153
Reviewed-by: Stefan Froemken
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters