docs: convert core and prover docs to mdbook project and add CI

[antonbaliasnikov]

What ❔

• Convert docs core project documentation to mdBook project
• Convert prover/docs prover project documentation to mdBook project
• Add CI workflows to build, test, and deploy versioned documentation for these projects using https://github.com/matter-labs/deploy-mdbooks action

Why ❔

Fixes #3185

To improve documentation visibility and usability in ZKsync era repository.

Tests

Deployed documentation examples are available in the fork:

• For core project: https://antonbaliasnikov.github.io/zksync-era/core/latest/
• For prover project: https://antonbaliasnikov.github.io/zksync-era/prover/latest/

Design decisions

It was chosen to use two different workflow files for documentation deployments to keep CI reliable and easy to support regardless of a bit of boilerplate code. Solution with the unified workflow for both projects was implemented but rejected as it is overcomplicated and more difficult to support in the long term.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zkstack dev fmt and zkstack dev lint.

[popzxc]

❤️