[llvm-mca][docs] Initial description of mca internals. NFC

This patch introduces a brief description of the components of MCA.  The main
focus is on Views.   This is a work in progress, and more descriptions will be
introduced later.  I want to flesh-out the Views section more and provide a
detailed description of eventing in MCA.  Eventually a brief code example of a
View should accompany the description.

Also, we should consider moving the MCA internals guide elsewhere at some point.

llvm-svn: 337219