[FTF-428] Add intro text to all pages in new nav

[m-hulbert]

Description

The PR adds an intro property to the frontmatter of all files to generate italics intro text. It also removes some unused frontmatter properties from the API references.

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch ftf-428-nav-intro-text

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