[Docs Site] Stringify vendored Markdown frontmatter as proper YAML

[KianNH]

Summary

Stringify vendored Markdown frontmatter as proper YAML

[hyperlint-ai]

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PR Change Summary

Stringified vendored Markdown frontmatter as proper YAML for improved consistency and parsing.

• Updated Markdown frontmatter to use proper YAML formatting
• Enhanced test fixture for Markdown generation
• Improved overall documentation clarity

Modified Files

• src/content/docs/style-guide/fixtures/markdown.mdx

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[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/style-guide/	@dcpena, @cloudflare/pcx-technical-writing
*.ts	@cloudflare/developer-advocacy, @kristianfreeman, @kodster28, @pedrosousa, @haleycode, @GregBrimble, @KianNH, @WalshyDev

[github-actions]

Preview URL: https://98ba7b73.preview.developers.cloudflare.com
Preview Branch URL: https://kian-pcx-18351.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/style-guide/fixtures/markdown/	https://kian-pcx-18351.preview.developers.cloudflare.com/style-guide/fixtures/markdown/