Move documentation to Github Pages

[Nashtare]

Turn the documentation into mdBook style and deploy to Github Pages
I should have taken care of all hyperlinks tweaks, but it's possible some slipped through.

More generally, given that now everything is under the monorepo, I think it'd be nice to extend the doc not just to the backend arithmetization but to the whole zk_evm stack, how the leader / workers are orchestrated, the whole data flow, etc...

[Nashtare]

Doc deployed at https://0xpolygonzero.github.io/zk_evm/.
I've updated README accordingly and (created +) updated https://0xpolygonzero.github.io/.

Once good to merge, I'll update the CI to be triggered only on pushes onto main / develop

[0xaatif]

Big fan of this!

I do wonder how much we can move to rustdoc.
In my experience, out-of-tree documentation is always at higher risk of getting stale.
It would be great to see these resources be tied to the code, with rust-managed hyperlinks etc.

[muursh]

Now the real fun begins.

[Nashtare]

@0xaatif I agree, there is a BIG todo list around the overall documentation, with several points that would need internal discussions first before addressing. We could probably open a discussion on that topic and start going through how we want our doc to look like, its covered area, etc.