Markdown content prop -> frontmatter prop

[bholmesdev]

What kind of changes does this PR include?

• New or updated content

Description

Change all references to the Markdown layout Astro.props.content property to Astro.props.frontmatter. Both will continue to work, but frontmatter should be the new recommendation. Seemed a bit much to add deprecation notices everywhere, so I'm voting for a clean sweep for 1.0 stable here!

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[yanthomasdev]

Did a scan through the docs Markdown files and I think you covered all the content occurrences, LGTM! Thanks for this @bholmesdev 🙌

[sarah11918]

@bholmesdev These look good, BUT, the content from integrations-guide/integrations/mdx is mirrored from https://github.com/withastro/astro/tree/main/packages/integrations/mdx/README.md so the changes have to be made there.

Otherwise, I think this is fine!

[bholmesdev]

@sarah11918 MDX README updates here!
withastro/astro#4237

[sarah11918]

@bholmesdev Got it! (See my comment in PTAL re: one other link you could fix on that page for me, while you're there. Or, I can do it before you merge.)

ARE YOU OTHERWISE OK HAPPY WITH THIS PR, and the changes to the other 2 pages?

Also there are conflicts to resolve that either your or I can take care of. They look pretty straightfoward to choose the right thing. TOO MUCH MD/MDX ACTIVITY.