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.

More pages will soon be added.
There should be a space between the icon and "icon". Looks weird otherwise http://picard.mbsandbox.org/docs/basics/#manual-lookup