[TASK] Show extension keys in documentation list

EXT:documentation now shows extension keys as well.

Change-Id: Ibe44884d7eb98d842ea10dd6cd91778630ecf7cf
Resolves: #52830
Relates: #52782
Reviewed-on: https://review.typo3.org/24777
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers