Docs: Remove Saga link

[sunker]

What this PR does / why we need it:

Follow-up from #2241 (comment). Also removing another section in this doc where the Saga website is being referenced.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[Copilot]

Pull Request Overview

This PR removes references to the Saga Design System from the plugin tools documentation. The Saga Design System link is being removed from two locations in the best practices documentation where it was previously referenced alongside Grafana UI components guidance.

Key changes:

• Removed Saga Design System reference and link from the frontend best practices section
• Removed entire bullet point about practicing good front-end design that referenced Saga
• Retained guidance about using Grafana UI components and their documentation

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[github-actions]

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