Functional `matchmaps.ncs` and more

[dennisbrookner]

The matchmaps.ncs utility is now functional and available in the command-line. Additionally, this PR includes:

• documentation for matchmaps.ncs on the CLI page of the docs
• A placeholder "about" page on the docs
• checks that the specified input directory exists, and a helpful error message if not, for both matchmaps and matchmaps.ncs
• wrap _unit_cell_hack into a utility, as it's now used multiple times