Render documentation using mdbook

[MegaRedHand]

We currently have some documentation files under docs/ that we could render into a website using mdbook. For that, we'd also need to add a SUMMARY.md.

For an example, see how ethrex does it: https://github.com/lambdaclass/ethrex/blob/728dc7ded560c665ab1ff2cf7f3eeb197b5bc40f/Makefile#L201. They also use some plugins that might be useful.