Minor errors of the website

[Felix Janda]

The w3c validator finds errors on http://lilypond.org/development.html
because the <thead> element is used multiple times in a single table.
In the source Documentation/web/community.itexi there are multiple
@headitems inside on @multitable. Splitting this into several
multitables should solve the errors but it might also change the
appearance.

On the same page (and some other pages) there are several links like
"../doc/v2.17/Documentation/contributor/index.html". Notice the dots in
the beginning. They don't really make sense. (The lynx browser complains
because of them.)

Felix