Summary:
Add user documentation page. This is an empty page atm, later patches will add
the specific user documentatoins.

Reviewers: dkrupp

Subscribers: whisperity, xazax.hun, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, gamesh411, Charusso, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64494

llvm-svn: 365639