Set up mdbook docs site (litmus-yesx)


Status	Done · `archived`
Type	Task
Priority	normal

Add mdbook documentation site with book.toml, src/ files, mise tasks, flake.nix integration, and .gitignore update

Summary of Changes

Created docs/book.toml (title: Litmus, src: src, build-dir: dist)
Created docs/src/SUMMARY.md with 4-page TOC
Created docs/src/introduction.md from README problem/solution section
Created docs/src/concepts.md with three-layer model and provider ecosystems
Created docs/src/milestones.md adapted from existing docs/milestones.md
Created docs/src/contributing.md placeholder
Added pkgs.mdbook to flake.nix devShells packages
Added docs-serve and docs-build tasks to .mise.toml
Added docs/dist/ to .gitignore
Verified: mdbook build docs produces docs/dist/ successfully