website margins

[Graham Percival]

Patrick, IIRC you recently changed the CSS to have bigger margins.
This wastes a lot (relatively speaking) of horizontal space on
small screens+windows.  Could you keep the smaller margins (more
text space) for small screens?  I don't mind if you do something
fancy and increase the margins for large windows.

My current approximate goal is to get all of the introduction +
regular use + infrequent manuals visible in a typical
(non-specially-tweaked) firefox window that's 1024x768.  Even
that's a bit larger than I'd like to target, but let's meet this
first goal before trying to get it even smaller.

I've been rewording manual descriptions (saving 2-3 words on
average) to reduce their size, but I'd like to see how far I still
need to go, and 40 more pixels of text width would help quite a
bit.

"make website" should now work in a normal builddir after doing
"make doc", so you can test exactly what'll be on lilypond.org.

Cheers,
- Graham