[llvm-mca][docs] Add instruction flow documentation. NFC.

Summary:
This patch mostly copies the existing Instruction Flow, and stage descriptions
from the mca README.  I made a few text tweaks, but no semantic changes,
and made reference to the "default pipeline."  I also removed the internals
references (e.g., reference to class names and header files).  I did leave the
LSUnit name around, but only as an abbreviated word for the load-store unit.


Reviewers: andreadb, courbet, RKSimon, gbedwell, filcab

Reviewed By: andreadb

Subscribers: tschuett, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D49692

llvm-svn: 338319