no more Documentation/index.html

[Graham Percival]

Some of you might have bookmarked
  out-www/offline-root/Documentation/index.html
as your local lilypond docs.  This file no longer exists; please
do a git pull -r, (and then make doc), and then use
  out-www/offline-root/index.html

I don't expect this to matter for the 2.14 website.  Once 2.13.11
is out, we'll find out what happens to the 2.13.11 online docs
with the old website.

Cheers,
- Graham