replace build system with generated table of contents

Gets rid of `doc/README.mdtoc` and its build script. Since GitHub.com
renders anchors for each heading, all we have to do is put a simple
table of contents into `README.md` itself, and everything will get
linked up nicely.

Pros of this approach:
* We don't have to point out to people not to edit `README.md` anymore
* We don't have to run the build script each time README gets edited

Cons of this change:
* The "chapter" numbers are lost. They were silly anyway.

`doc/mdtoc` renders a Markdown table of contents for a Markdown file.
`doc/filter-toc` filters that down to only headings after ToC.
This script can be used to easily insert ToC into the current document
when editing `README.md` with, e.g., Vim:

    :read !doc/filter-toc %

doc/filter-toc

@@ -0,0 +1,4 @@
+#!/bin/bash
+# Render Markdown ToC with only headings appearing after "Table of Contents"
+dir="$(dirname "$0")"
+"$dir"/mdtoc "$1" | sed -n '/table-of-contents/,$p' | grep -v table-of-contents