docs: Reorganize landing page.

I clustered the docs by some intuitive/fuzzy notion of "similarity", and
some reasonable categories seemed to materialize. I tried to give the
clusters useful names, but you may want to take a look at the landing
page (<http://clang.llvm.org/docs/> for the lazy) and share your
thoughts.

I have to say, this small change really gives the docs a whole new life!
It makes our documentation quite a bit easier to navigate and scope out.

llvm-svn: 171379