Fix mangled code blocks

[jslatane]

Currently, there are many MDX examples that look like they should be code blocks but are single-line code segments separated by blank lines. This is not good for readability, and they should be combined in a true fenced code block.

[prmerger-automator]

@jslatane : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit dc93395:

✅ Validation status: passed

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docs/mdx/avg-mdx.md	✅Succeeded
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[Jak-MS]

@kfollis
Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team