Bug: Anchor link URL can be `#undefined`

[lfdebrux]

Originally reported in #305.

a link was being injected with the href of #undefined due to a missing id attribute on the <h2> element

What should change

This should be fixed at build time. Either we should autogenerates a sensible default, or failing that, report missing ids.

User need

End users need to have working anchor links for all headings. Authors need to know if a heading is missing a working anchor link.

[jamescarr28]

This seems to happen when users are writing in raw html . I think a linter check would be an appropriate way to resolve this rather than trying to force a default a user might not expect. We can also highlight using markdown syntax as an easier way to manage this