docs: add closeModal() API documentation for embed instructions

[hariombalhara]

docs: add closeModal() API documentation for embed instructions

Summary

Adds comprehensive documentation for the new closeModal() API method introduced in cal.com PR #24746. This method allows developers to programmatically close modal-based embeds (pop-ups and floating buttons) instead of using the undocumented internal hack.

Changes:

• Added new "Close modal" section to embedding/embed-instructions.mdx with full API documentation
• Added practical example in embedding/embed-events.mdx showing how to close modal after successful booking
• Included both namespaced and non-namespaced usage examples
• Documented supported embed types, error handling, and migration path from internal API

Review & Testing Checklist for Human

• Test the closeModal() API with actual embeds - Verify that Cal("closeModal") actually works with pop-up and floating button embeds as documented
• Verify the anchor link works - Check that the cross-reference link from embed-events.mdx to #close-modal in embed-instructions.mdx renders correctly on the help site
• Confirm API details match implementation - Double-check against cal.com PR #24746 that the API name, error message, and behavior are accurately documented
• Test the example code - Copy the example code snippets and verify they work in a real embed scenario (especially the bookingSuccessfulV2 callback example)

Test Plan

1. Create a test page with a pop-up embed (element click or floating button)
2. Add the bookingSuccessfulV2 event listener with closeModal() callback
3. Complete a test booking and verify the modal closes automatically
4. Try calling closeModal() on an inline embed and verify it throws the documented error
5. Test with both namespaced and non-namespaced embed configurations

Notes

• Documentation is based on code inspection of cal.com PR #24746, not actual testing
• The API was renamed from close() to closeModal() during development - confirmed the final name from the PR diff
• Examples use bookingSuccessfulV2 (not the deprecated bookingSuccessful event) per existing help docs
• All syntax follows existing help repo conventions (capitalized Cal(), proper MDX formatting)

Link to Devin run: https://app.devin.ai/sessions/8b4ff4e64f8a465384db398b61ac7f2b
Requested by: @hariombalhara (hariom@cal.com)

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