Docs: add dedicated Globals API page

[tharani-2006]

Closes #32380

What I did

Added a dedicated Globals page to the API documentation to give globals first-class visibility alongside parameters.

The new page explains what globals are, how they differ from parameters, and how to use them across preview, manager, and MDX contexts, with guidance on common pitfalls.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

No manual testing required.
This PR is documentation-only and does not affect runtime behavior.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Notes

• Adds a dedicated Globals page to the API section
• Documents globals usage in preview, manager, and MDX contexts
• Uses CodeSnippets for all examples (no inline code blocks)
• Improves clarity and discoverability compared to existing docs

Thanks!

[tharani-2006]

Hi 👋,

PR is ready for review. I’m happy to iterate on any feedback or requested changes.

Thanks!