docs: Replace static page with mdBook site

Uses mdBook[1] to generate a documentation overview page instead of
the previous HTML site.

This makes it possible to add more elaborate documentation without
having to deal with finicky markup.

[1]: https://github.com/rust-lang-nursery/mdBook

tools/nixery/docs/src/SUMMARY.md

@@ -0,0 +1,4 @@
+# Summary
+
+- [Nixery](./nixery.md)
+- [Nix, the language](./nix-1p.md)