- improve the tables generator(style/simplify)
  - show the differences between the compilers
  - add an explanation about interrelated CPU features

Add documentation for the new build infrastructure and API developed to enable
universal intrinsics.  Written by @seiko2plus with some fixes by @mattip.


* DOC: add SIMD optimization doc (seiko2plus)

* DOC: reformat as valid RST

* trim whitespace

* first part of Understanding CPU Dispatching

* update build options and remove implied features, gonna update it later

* add more explanations for the dispatcher and fix doc style

* fix up style

* add figure

* Improve and more explanations for Understanding CPU Dispatching

* fix up syntax

* DOC: tweak formatting

* DOC: more tweaks

* fix rst formatting

* DOC: Generate CPU features tables from CCompilerOpt

* DOC: move files around

* DOC: add comment to top of file

* DOC: rebuild tables, fix links

* DOC: minor copyedits

Co-authored-by: Sayed Adel <seiko@imavr.com>
Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>