Docs: Document group property for link extensions

[mckn]

Summary

Documents the new group property that replaces the deprecated category property on link extensions in Grafana 13. Includes comprehensive migration guidance with code examples and updates the UI extensions reference table.

What this PR does / why we need it

The group property provides improved control over how links are grouped and displayed in the UI, offering optional icon support compared to the string-based category property. This PR documents the migration path for plugin developers.

Related to: grafana/grafana#116481

Which issue(s) this PR fixes

Fixes #

Special notes for your reviewer

• The category property continues to work (backward compatibility) but will be removed in a future Grafana version
• Migration examples show usage in both direct config and the configure() callback
• The reference table has been updated to mark category as deprecated and document the new group property

[github-actions]

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✨ This PR can be merged. It will not be considered when calculating future versions of the npm packages and will not appear in the changelogs.

[Copilot]

Pull request overview

Documents the new group property for link UI extensions in Grafana 13 and provides migration guidance for plugin developers transitioning from the deprecated category property.

Changes:

• Updates the UI extensions reference to mark category as deprecated and document the new group object shape.
• Adds a Grafana 12.x → 13.x migration guide section explaining how to migrate from category to group, including examples.
• Updates the UI extensions how-to guide example comments to include group and flag category as deprecated.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docusaurus/docs/reference/ui-extensions-reference/ui-extensions.md	Updates the addLink() config reference table to deprecate category and add group.
docusaurus/docs/migration-guides/update-from-grafana-versions/v12.x-v13.x.md	Adds migration guidance and examples for moving from category to group.
docusaurus/docs/how-to-guides/ui-extensions/register-an-extension.md	Updates inline guidance/comments to reflect category deprecation and introduce group.

[hugohaggmark]

Awesomeness 🚀