[XRay][Docs] Add documentation for XRay in LLVM

Summary:
This is the initial version of the documentation for how to use XRay as
it stands in LLVM, Clang, and compiler-rt. We leave some room for later
expansion mentioining what is work in progress and what could be
expected moving forward.

We also give a high level overview of future work that's both ongoing
and planned.

Reviewers: echristo, dblaikie, chandlerc

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D26386

llvm-svn: 286319