[skip ci] Integrate CV32A6 Design Document and break out CVA6 APU

I have a task to translate Jérôme's User Manual into ReStructured Text (rst) so it can be incorporated into the User Manual published on ReadTheDocs (link). Currently the CVA6 UM is a hierarchical document comprised of three separate documents:

1. The OpenHW Group CVA6 User Manual. This is largely re-used from Ariane and most of the content was produced by Florian and Nils.
2. The CVA6 Requirement Specification. This document is exactly what you'd expect from the title. Jérôme is the editor of this document.
3. The CV32A6 Design Document. This document is still under development and will eventually capture the micro-architecture of the CV32A6.

So we have two "User Manuals", the original OpenHW Group CVA6 User Manual, and "Jérôme's User Manual". Also, it seems to me that the document currently called a User Manual is not a User Manual. Rather it is a combination of a CVA6 micro-archiecture specification and the CORE-V APU (CVA6 SoC) user manual. So I am proposing a few changes:

1. As planned, convert Jérôme's User Manual into a set of rst files and use them to define theOpenHW Group CVA6 User Manual.
2. Leave the CVA6 Requirement Specification as is.
3. Add CVA6-core specific content from the original OpenHW Group CVA6 User Manual to the CV32A6 Design Document.
4. Create a new document called the CVA6 System-on-Chip which would be extracted verbatium from that chapter in the current OpenHW Group CVA6 User Manual.

This pull-request implements the above.