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.
Created a dropdown menu to show doc pages
A general endpoint for all doc templates
Added the FAQ page

from: https://wiki.musicbrainz.org/MusicBrainz_Picard/FAQ
Added Linux installation instructions

from: https://wiki.musicbrainz.org/Picard_Linux_Install
Added UI Options

from: http://wiki.musicbrainz.org/MusicBrainz_Picard/Documentation/Options
Added Tags documentation

from: http://wiki.musicbrainz.org/MusicBrainz_Picard/Tags
Added the Scripting doc

from: http://wiki.musicbrainz.org/MusicBrainz_Picard/Documentation/Scripting
Added the Illustrated guide

from: http://wiki.musicbrainz.org/How_To_Tag_Files_With_Picard

This page will have to be updated once Picard 1.3 is released as the
screenshots are old.
Added Troubleshooting page

from: wiki.musicbrainz.org/MusicBrainz_Picard/Troubleshooting
Added Development related pages

The text for the /development/ page has been copied from:
http://wiki.musicbrainz.org/MusicBrainz_Picard

The Build on Windows page comes from:
http://wiki.musicbrainz.org/How_to_Build_Picard_for_Windows

And the Build on OSX (created by bitmap) comes from:
wiki.musicbrainz.org/User:Bitmap/Building_Picard_on_OS_X
Added examples of the Picard Plugin API

from: http://wiki.musicbrainz.org/MusicBrainz_Picard/Plugins/API

Every line of <pre> tags has been numbered via CSS (and some JS.)

Future improvements might be to have a button that copies the code,
and some Python syntax highlight.
Added Tag mappings

from: wiki.musicbrainz.org/MusicBrainz_Picard/Tags/Mapping

This is the only page that still needs some work, as the table is HUGE
and needs to be scrolled horizontally, but I can't think of anything
that might improve readability.
Point to musicbrainz/docs rather than the wiki
Minor tweaks as suggested by @Sophist-UK
Merge remote-tracking branch 'origin/docs'

Conflicts:
	templates/base.html