Draft: PoC: isaspec: Add support to generate documentation

What does this MR do and why?

⚠ This is a PoC that I would like to use for discussion ⚠

I have seen https://gitlab.freedesktop.org/panfrost/valhall-docs and I like it to have some kind of auto generated ISA docs based on isaspec.

I am not sure if there is interest for something like this, so that's why I uploaded this patch.