lilypond-manuals.css: Add a maximum width for manuals sidebar

lilypond-manuals.css: Add a maximum width for manuals sidebar

When the browser window is 1280px wide or more, the sidebar
will be 346px wide, rather than a percentage of the window
width.

Also contains these two commits:

lilypond-manuals.css: A better brown for the usage manual

I think this shade goes better with the other colors,
giving us a more coherent color scheme across manuals.

lilypond-manuals.css: Add space between top-levels in sidebar

(This one was originally in my last patch for the manuals css,
but I found a better way to do it, so removed it from the
previous patch and including it here.)

[pwm, 2017-06-17 22:04:48]

Please review, a few follow-ups to the previous manuals css patch.  

The following demo site reflects the changes in this patch (remove spaces after
the domain).

http://clairnote.org   /lilypond-web-demo/Documentation/

(I'll upload a screenshot showing the former brown color on the issue tracker.)

Thanks,
-Paul