Transition documentation to commonmark

There has been talk about transitioning the ibex documentation into a similar format to that used in OpenTitan (OT) . The motivation is that, once vendored into the OT repository, the ibex documentation could be included in the larger OT documentation.

The format used in OT endeavors to be standard commonmark with only the GitHub Tables, so should work with a number of different site generators. However, it would make sense for ibex to use the same generator as OT (mdbook) for it's own stand-alone documentation generation.

Start by inventorying the Ibex documentation and comparing its current format with OpenTitan's CommonMark and GitHub Tables conventions. Review the mdBook documentation for standalone generation. Done means Ibex documentation can be vendored into OpenTitan and generated independently with mdBook.