Added a basic documentation page

from: https://musicbrainz.org/doc/MusicBrainz_Picard/Documentation

The doc page design (especially the sidebar) and code has been
copied, like most of this site, from the awesome bootstrap website:
http://getbootstrap.com/components/

A note has been added to the footer, which can quickly take anyone to
the file on Github, so they can edit the doc.