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/book.toml

@@ -0,0 +1,8 @@
+[book]
+authors = ["Vincent Ambo <tazjin@google.com>"]
+language = "en"
+multilingual = false
+src = "src"
+
+[output.html]
+additional-css = ["theme/nixery.css"]