the docs need a link back to the main directory

[Mark Polesky]

Once a user navigates into the docs from the main directory...

   http://lilypond.org/doc/v2.11/Documentation/

... there's no link back, so if I'm in NR and I want to go to IR, 
I'mforced to either a) keep pressing the back button or b) highlight
and delete stuff from the address bar.

If I'm in NR and I click the "Top" link, I frustratingly get the top
of NR, not the top of the entire Documentation, and there's no "Up"
link from there.

I know I mentioned this before*, but even with all the fantastic work 
everyone's done, little things like this are frustrating, especially
to potential future LilyPond users, who we don't want to lose.

Thanks,
Mark

* http://lists.gnu.org/archive/html/lilypond-user/2008-09/msg00715.html