Migrate "manual" style documentation to mdbook.

We discussed this today. AIUI, the initial plan is:

• Leave introduction.md in place for now, but plan to migrate pieces of it later.
• Create a new book top-level directory with an mdbook instance in it. Start writing the book.
• Leave reference.md alone permanently.
• Figure out how to host the rendered book at some point.

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Revised plan (15 May):

1. Create "book" with an mdbook instance. !350 (merged)
2. Add CI support for building the mdbook instance. !350 (merged)
3. Cause gitlab to deploy the book whenever it is merged to main. !350 (merged)
4. Robustness improvements to mdbook-build and related scripts !371 (merged)
   • Add usage documentation/comments
   • Reject unknown options
5. Move text from introduction.md to mdbook. !372 (merged)
   • And add building/editing instructions to HACKING.md
   • And add appropriate cross-links from docs.rs documentation to mdbook.
   • And rename from "introduction" to guide.
6. Add intermediate index.html files to generated page.

At this point, I would prefer to just work on the documentation for a while.

Later we would want to:

8. Add facilities to build local link-adjusted versions of the mdbook and the rustdoc, and to validate that the links are sensible.
9. Purchase a domain to use instead of diziet.pages.torproject.net; add a redirect or a vhost.
10. Arrange for per-tag versions of the book to exist and link to appropriate corresponding versions of the rustdoc.
11. Combine any duplicated logic wrt link checking with torspec mdbook code