Deprecating the "Authoring markdown" section and adding config docs for the new legacy flag

[tony-sull]

What kind of changes does this PR include?

Updating the docs to reflect the new "legacy" mode for Astro-flavored markdown in PR #4016

Description

• Deprecates the "Authoring markdown" section with a note on how to re-enable the features until they are removed in a future Astro release (no timeline on that, I'd guess 2.0?)
• Adds configuration docs for the new legacy.astroFlavoredMarkdown flag

[netlify]

✅ Deploy Preview for astro-docs-2 ready!

Name	Link
🔨 Latest commit	8fa70ee
🔍 Latest deploy log	https://app.netlify.com/sites/astro-docs-2/deploys/62de8eb5b135c8000ab28a9e
😎 Deploy Preview	https://deploy-preview-1090--astro-docs-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[sarah11918]

Thanks for this, @tony-sull !

Two related notes:

1. The configuration-reference page is auto-generated from @types/astro.ts in the main repo, so we don't make changes to it here on our end. Please make sure these changes are made in the astro repo, if you haven't already, and you can just discard them here.

2. Do you also want to add content at this time for the migration guide page? Or would you like that "on the stack"? (I believe we already have a note in an issue we're monitoring for Migration Guide content that it's on our radar to expect.)

[tony-sull]

🤦 I always forget the Configuration Reference is auto-generated, the docs project is too fancy for me! 😅

Just removed the manual config reference updates, I'll go ahead and take care of the migration guide while I'm here!

[sarah11918]

Note: this link check is going to fail until the changes from astro.ts are merged, and then make their way here. So, the PR is fine, but I'll hold off on this until the link to the new configuration flag exists!

[sarah11918]

This one now depends on withastro/astro#4027 with updated content for the configuration-reference page.

Once that merges, and the next day the changes make it into docs, then we can double-check the anchor link for the new option, and this should good to go.

[sarah11918]

@tony-sull This looks like it passes the link checker now that the legacy flag exists on the configuration reference page, so should be good to merge!