(tech debt) Remove HEADER_SPAN hack

[Chriscbr]

The markdown renderer has a small hack when rendering headers that allows the "HEADING_SPAN" environment variable to be passed to replace all anchors with <span> tags with data-attribute properties. I think this hack exists so that anchors can be rendered properly on Construct Hub using the prism-react-renderer component.

Once the Construct Hub has switched to rendering API references directly from API.json files, we can remove this hack.

[github-actions]

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[mrgrain]

I think we are still blocked here.