org mode allows rendering the toc anywhere in the html document using the `TOC`
keyword [1]. There's more options but `#+TOC: headlines $n` should be enough
for starters. Note that org mode still requires setting `#+OPTIONS: toc:nil` to
disable the default toc
[1] https://orgmode.org/manual/Table-of-Contents.html
Until now we expected the .org file to print back to itself - we can't do that
when the input is not pretty printed already - with the introduction of blocks
with unindented content that will be the case.
