Add docs for renderMarkdown

[ascorbic]

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Adds docs for the new renderMarkdown helper provided to content loaders

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en/reference/content-loader-reference.mdx	Source changed, localizations will be marked as outdated.

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

Looks great, Matt!

I had one suggestion for, as usual, breaking up a paragraph. 😄 For your consideration!

[sarah11918]

Thank you for your patience, Matt! This can now go live with the other 5.9.0 docs! 🥳