Import Crosslink design & analysis from hackmd pages.

[nathan-at-least]

This transplants all of the Crosslink hackmds into this mdbook which satisfies #85.

Goals: make minimal changes to the hackmd content in order for rendering and crosslinking to be coherent.

Not goals of this PR: editorial changes, structural changes, styling changes (beyond supporting existing markup rendering).

• [x] Restructure the Design chapter to house all of the Crosslink docs.
• [x] Copy the Crosslink hackmd pages into distinct subchapters of src/design/crosslink/.
• [x] Add a brief sub-table-of-contents at src/design/crosslink.md. This page should probably have better intro text in future PRs.
• [x] Set up mdbook-admonish as a dependency in book.toml, the README, the github workflows, and the nix flake.
• [x] Update all mdbook-style ::: blocks to use mdbook-admonish and verify they render correctly.
• [x] Fix all intra-doc links to point into the mdbook content (rather than external hackmd content).
• [x] Fix a few other rendering differences between the two. For example mdbook does not render **foo**.