[Markdoc] headings and heading IDs

[bholmesdev]

Changes

• Resolves Getting headings from a markdoc file using the render() method returns an empty object. #6855
• Generate heading IDs for all Markdoc headings
• Populate the headings array from the render() function
  • Create a new @astrojs/markdoc/runtime export that exposes a collectHeadings utility
  • Add a getHeadings() function to all .mdoc modules. This approach will call transform twice (once for the Content component, once for getHeadings()). This is due to a fundamental limitation of allowing Markdoc variables as component props. A TODO for a future exploration is noted in the source code.
• Expose a defaultNodes object to use when modifying nodes from the Markdoc config

Testing

• Test heading IDs and TOC
• Test rendering component Astro components

Docs

Update Node headings example in the README. In the future, we may need to document each node case-by-case!

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Latest commit: 11c388f

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[ematipico]

Let's ship it! 🚢

[sarah11918]

Just a quick heads up @bholmesdev that I literally saw this merge as I was looking at the README! Reminder for next time! ❤️

[sarah11918]

Late to the party (whomp, whomp) but README docs are great! 🎉

[bholmesdev]

@sarah11918 CRAP I 100% forgot to tag you on this 😓 Assumed we'd reviewed it with all the docs PRs flying around. If you want to leave comments here for me to address in a fast follow, happy to do so

[sarah11918]

Sorry @bholmesdev - I was prioritizing the specific "merge on release" docs I'd been given (and, didn't know this Markdoc update was something to be released for 2.5) so I assumed this was a "regular" PR, and was just clearing out all the Astro Docs Maintainer pings now. Docs are great though! No worries!

[bholmesdev]

It is a "regular" PR, was just itching to merge to align Markdoc 0.2.0 with some data collection changes. I'll check next time, my bad!