[libc] Update look and feel of libc.llvm.org

This design is borrowed from the lldb folks (thank you!) to declutter
the page.

 * The version number at the top is removed.
 * Links are pushed over to a sidebar
 * The sidebar has headings

There are other minor changes:

* The warning about this project not being ready is now an RST "warning"
* Links to the Bug Reports and the Source Code are Added
* Refer to this project as either "The LLVM C LIbrary" or "The libc"

Tested:
Built locally

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D131242