`stable` and `dev` channels

[XVilka]

Currently, we have only one channel - master, which is deployed to https://book.rizin.re. I suggest:

• Use the same branching scheme as other Rizin projects - stable and dev
• Deploy stable on https://book.rizin.re/
• Deploy dev on https://book-dev.rizin.re/ or a similar domain
• See .github/workflow/mdbook.yml for branch settings (also update the base)
• Update Rizin release checklist issue template to include the step of tagging the book too (optional?)
• Point Rizin site to two version of the books - stable and dev