Docs: Add Learn more link to Interactivity API reference

[Sakshi-geeky]

What?

Adds a "Learn more" link to the Interactivity API reference documentation page, providing easy access to the main Interactivity API documentation.

Fixes #70390

Why?

This improvement helps developers quickly navigate to the main Interactivity API documentation from the API reference page, improving the documentation experience.

How?

• Added a "Learn more" link below the main heading in docs/reference-guides/interactivity-api/api-reference.md
• The link points to the main Interactivity API README file

Testing Instructions

1. Navigate to the Interactivity API reference documentation
2. Verify that a "Learn more" link appears below the main heading
3. Click the link to ensure it correctly navigates to the main Interactivity API documentation

[github-actions]

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[github-actions]

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Unlinked contributors: Sakshi-geeky, chefranov.

Co-authored-by: talldan <talldanwp@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>
Co-authored-by: andreiglingeanu <andreiglingeanu@git.wordpress.org>

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[github-actions]

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