Suggestions: Add plugin developer docs

[fastfrwrd]

Updates the Plugin author docs to have accurate and complete information on how to configure Suggestions for a panel plugin.

[github-actions]

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[Copilot]

Pull request overview

This PR updates the Grafana Plugin Tools docs to provide current, panel-plugin-focused guidance for implementing visualization suggestions (the “Suggestions” / visualization picker integration), and removes the older Angular→React migration doc for suggestion suppliers.

Changes:

• Adds a new how-to guide that explains how to enable and implement a suggestions supplier for panel plugins.
• Updates existing docs to link to the new guide and introduces a “Next steps” section in the panel tutorial.
• Removes the obsolete Angular→React migration guide page for suggestion suppliers and updates the sidebar accordingly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
docusaurus/website/sidebars.ts	Adds the new suggestions guide under Panel plugins and removes the old Angular→React entry.
docusaurus/docs/tutorials/build-a-panel-plugin.md	Adds “Next steps” links, including the new suggestions guide.
docusaurus/docs/reference/metadata.md	Expands plugin.json suggestions field description with a link to the new guide.
docusaurus/docs/migration-guides/angular-react/suggestion-supplier.md	Deletes the outdated migration guide page.
docusaurus/docs/key-concepts/anatomy-of-a-plugin.md	Adds a “Visualization suggestions” concept section pointing to the new guide.
docusaurus/docs/how-to-guides/panel-plugins/add-suggestions-support.md	New end-to-end documentation for implementing and testing suggestions suppliers.

[urbiz-grafana]

Left a few style suggestions, LGTM otherwise!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 3 comments.

[cla-assistant]

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1 out of 2 committers have signed the CLA.

✅ fastfrwrd
❌ adela-almasan
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ fastfrwrd
❌ adela-almasan
_{You have signed the CLA already but the status is still pending? Let us recheck it.}