Take 2: Create a working mdbook environment.

This MR replaces !344 (closed). It does not migrate any text to mdbook; it only creates an mdbook environment to build in.

Changes since the last branch:

• Use build.rs for test automation.
• Rename book tests package.
• Add mdbook-linkcheck and linklints support (copied from torspec)
• Use CI integration so that every merge to main causes a deployment.
• No actual text is in the new mdbook.

To see an example of the built book: https://nickm.pages.torproject.net/rust-derive-adhoc/

The final URL will probably be https://diziet.pages.torproject.net/rust-derive-deftly/

This branch is based on !352 (closed) to disable the every-commit job. I can remove that as needed; or feel free to rebase to get rid of it before merging.

Also see #66 for a checklist and mini-roadmap for this project.