[MLIR][SPIRVToLLVM] Updated documentation on spirv-cpu-runner

This patch adds documentation for the `mlir-spirv-cpu-runner`.
It provides an overview of applied transformations and passes, as
well as an example walk-through.

Some typos in the documentation have been fixed as well.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D93620