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 %
