docs: add changelog entry for If component products and versions props

[sbawabe]

Summary

Adds a changelog entry documenting the new products and versions props for the <If> component. This feature allows documentation authors to conditionally show content based on where a page is located within the navigation structure (which product or version the user is viewing).

This changelog entry accompanies the implementation PR: https://github.com/fern-api/fern-platform/pull/6456

Review & Testing Checklist for Human

• Merge order: This changelog should be merged after the fern-platform implementation PR (#6456) is merged, otherwise users will see documentation for a feature that doesn't exist yet
• Link validity: Verify the link /learn/docs/authentication/rbac#access-control-within-mdx-pages works correctly (note: the RBAC page currently only documents the roles prop, not the new products/versions props)
• Preview the changelog: Check that the changelog entry renders correctly in the docs preview

Notes

Link to Devin run: https://app.devin.ai/sessions/eea1536f18554cde8cebf3db3898a470
Requested by: Sarah Bawabe (@sbawabe)

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