Increase the depth of TOC

[falseresync]

Currently the TOC is pretty useless on some pages, for example on the page for codecs it's just 3 links whereas there are many subsections which you might want to jump to.

I suggest setting markdown.toc.levels option to something higher, like [2, 3] or [2, 3, 4]

[IMB11]

I'll take a look at this later today.