Some of the Markdown "spec" docs are getting big enough to warrant a TOC.

We already support a TOC in tool guides (i.e. man pages), meaning that the appropriate CSS is already in place. All we need is to specify the `--toc` option when we run pandoc.

However, some spec docs already have a hand-written TOC, and others will need their headings to be adjusted before they can use an auto-generated TOC. Therefore, it would be good to have an opt-in mechanism for a TOC for those docs that want it.