This PR updates the Interactivity API documentation to rename "API Reference" to "Directives and Store API Reference" and renames the file from api-reference.md to directives-store-api-reference.md as requested in WordPress#61207.

What?

Closes WordPress#61207

• Renamed heading in the Interactivity API docs.
• Renamed the file to prevent future conflicts and follow the naming convention.

Why?

To make the documentation clearer, align with Reference Guides structure, and avoid potential filename conflicts in the future.

How?

Edited the Markdown file on GitHub, updated the heading, and renamed the file.

Testing Instructions

1. Open docs/reference-guides/interactivity-api/directives-store-api-reference.md.
2. Confirm the heading reads "Directives and Store API Reference".
3. Check internal links pointing to this file work correctly.

Testing Instructions for Keyboard

• Navigate the file using the keyboard and ensure the heading has been updated properly.