docs: Reorganize landing page.

Language extensions are highly relevant to using clang as a compiler, so
move LanguageExtensions up into `Using Clang as a Compiler` on the
landing page.

The other documents from the now-gone `Language Extensions and Specs`
section on the landing page nicely fit hierarchically under
LanguageExtensions.rst, so put them under LanguageExtensions.rst's
toctree instead of on the landing page.

Impetus from Jordan Rose.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171409 91177308-0d34-0410-b5e6-96231b3b80d8