Support 2 levels of headings in Table of Contents

This issue was forked from #61.

Only including 2nd-level links in the TOC is very limiting, especially for a page such as the release notes. Yet allowing for 3rd-level links brings us back to the issue of duplicate text headers not becoming links. I feel trapped in a small box. Do you have a suggestion as to how the release notes can be displayed? Users complain of not having a decent TOC for such a document. They want to see quickly what new features have appeared and this can be accomplished, in principle, by a TOC, especially since this is what was available in the [old MAAS docs](https://docs.ubuntu.com/maas/1.9/en/changelog).