table of content syntax should be {:toc} for compatibility with Kramdown

[thomas-sittler]

See: https://kramdown.gettalong.org/converter/html.html#toc

[astoilkov]

We are currently considering how to handle all TOC syntaxes. Tt seems there are a lot of variations and users want what they are used to.

Thanks for the suggestion. We will see what we can do.