Add MxN documentation to plugin manuals list

Summary: Now the MxN documentation (that is also used as the F1 help page) is shown in the list with the other plugins.

Test Plan: Build the documentation, look if under MITK User Manual -> MITK Plugin Manuals there is an entry for the MxN Display

Reviewers: kislinsk

Reviewed By: kislinsk

Differential Revision: https://phabricator.mitk.org/D806