[BUGFIX] EXT:documentation should accept other index file names

EXT:documentation checks for Index.html, index.html and index.htm as
main files for additional html documentations.

Change-Id: Idd2970ef9d6cc1b6f436b0c0fe0c607cf7b9d903
Resolves: #52569
Releases: 6.2
Reviewed-on: https://review.typo3.org/24441
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn