Configurable table of contents

[raqbit]

Currently we're using the markdown-it-toc-done-right plugin to automatically generate a table of contents whenever [toc] is found in the document.

This works fine, but there are some options in the TOC plugin which the user might want to change, like whether the document title should be in the TOC or if certain anchor-only headers should be listed there. Some of these things could be added as a config option to docmaker.yaml, but others will require changes to the TOC plugin. When working on that, we might as well look into adding these options as some sort of syntax change instead.