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

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

tools/nixery/docs/src/nix-1p.md

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+This page is a placeholder. During the build process, it is replaced by the
+actual `nix-1p` guide from https://github.com/tazjin/nix-1p

tools/nixery/docs/src/nixery.md

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+![Nixery](./nixery-logo.png)
+
+------------
+
+Welcome to this instance of [Nixery][]. It provides ad-hoc container images that
+contain packages from the [Nix][] package manager. Images with arbitrary
+packages can be requested via the image name.
+
+Nix not only provides the packages to include in the images, but also builds the
+images themselves by using an interesting layering strategy described in [this
+blog post][layers].
+
+## Quick start
+
+Simply pull an image from this registry, separating each package you want
+included by a slash:
+
+    docker pull nixery.dev/shell/git/htop
+
+This gives you an image with `git`, `htop` and an interactively configured
+shell. You could run it like this:
+
+    docker run -ti nixery.dev/shell/git/htop bash
+
+Each path segment corresponds either to a key in the Nix package set, or a
+meta-package that automatically expands to several other packages.
+
+Meta-packages **must** be the first path component if they are used. Currently
+the only meta-package is `shell`, which provides a `bash`-shell with interactive
+configuration and standard tools like `coreutils`.
+
+**Tip:** When pulling from a private Nixery instance, replace `nixery.dev` in
+the above examples with your registry address.
+
+## FAQ
+
+If you have a question that is not answered here, feel free to file an issue on
+Github so that we can get it included in this section. The volume of questions
+is quite low, thus by definition your question is already frequently asked.
+
+### Where is the source code for this?
+
+Over [on Github][Nixery]. It is licensed under the Apache 2.0 license. Consult
+the documentation entries in the sidebar for information on how to set up your
+own instance of Nixery.
+
+### Which revision of `nixpkgs` is used for the builds?
+
+The instance at `nixery.dev` tracks a recent NixOS channel, currently NixOS
+19.03. The channel is updated several times a day.
+
+Private registries might be configured to track a different channel (such as
+`nixos-unstable`) or even track a git repository with custom packages.
+
+### Is this an official Google project?
+
+**No.** Nixery is not officially supported by Google.
+
+### Should I depend on `nixery.dev` in production?
+
+While we appreciate the enthusiasm, if you would like to use Nixery in your
+production project we recommend setting up a private instance. The public Nixery
+at `nixery.dev` is run on a best-effort basis and we make no guarantees about
+availability.
+
+### Who made this?
+
+Nixery was written mostly by [tazjin][].
+
+[grahamc][] authored the image layering strategy. Many people have contributed
+to Nix over time, maybe you could become one of them?
+
+[Nixery]: https://github.com/google/nixery
+[Nix]: https://nixos.org/nix
+[layers]: https://grahamc.com/blog/nix-and-layered-docker-images
+[tazjin]: https://github.com/tazjin
+[grahamc]: https://github.com/grahamc