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/theme/nixery.css

@@ -0,0 +1,3 @@
+h2, h3 {
+  margin-top: 1em;
+}