docs(docs-infra): fix aria-label parsing breaking headings

[SkyZeroZx]

Strip all HTML tags and escape quotes in aria-label to avoid breakage

What is the current behavior?

See https://next.angular.dev/ai/ai-tutor#phase-5-experimental-signal-forms--warning-subject-to-change

What is the new behavior?

[leonsenft]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 6d2eed6
• 21.2.x: 8818095

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