Transition documentation to commonmark

[HU90m]

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.