[Beta] Infer headings at runtime

[gaearon]

It seems like we've ripped out a piece of logic that generated headings automatically because they're taken from the annotation. This is a bit annoying because in development, you can't use a header link until you've rerun yarn fix-headings. We should re-add this logic back so that headings still get automatic headers. We still use the precommit hook to force their generation, but the implicit version is helpful when writing content.