User Manual re-formatted as ReStructuredText

Hi Davide. This PR has a lot of changes, but it all boils down to translating the user_manual to RST and creating Sphinx builds so that readthedocs will render it all into html and pdf. The verification strategy is also available in rst.

The original MSWord version of the user_manual is still there (look in cores/cv32e40p/user_manual/origin), so no information is lost.

Some of the tables need to be reviewed, but that can be done later.