[FEATURE] Use mod dispatcher for opendocs extension

Use the backend module dispatcher for the
opendocs extension.

Added an index.php that redirects to
alt_doc.php which contains the logic.

Change-Id: I2a939cfe5f48d760718a0ae330abdc0e75cda135
Resolves: #38102
Releases: 6.0
Reviewed-on: http://review.typo3.org/12109
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel