Support 2 levels of headings in Table of Contents

[pmatulis]

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.

[nottrobin]

I think this is a reasonable argument for including one more level of headings in the TOC. But I'm not sure it should be the default option, and we will also need to solve the duplicate heading issue.

[pmatulis]

Can we not allow the source file (.md) to select a second-level TOC?