tidb-cloud: add TopRU user guide in TiDB Cloud Clinic (Premium)

[zimulala]

What is changed, added or deleted? (Required)

This PR adds full TopRU user documentation for TiDB Cloud Clinic (Premium/Dedicated scope), instead of only adding a link.

Changes include:

• Add a new doc page: tidb-cloud/premium/top-ru-in-clinic-premium.md
  • Feature overview and recommended scenarios
  • Prerequisites
  • Page views and usage workflow
  • Observation dimensions and granularity
  • Limitations and FAQ
• Update navigation:
  • TOC-tidb-cloud-premium.md
  • tidb-cloud/premium/_index.md
• Update tidb-cloud/tidb-cloud-clinic.md to include TopRU entry and section.
• Add TopRU screenshots under media/tidb-cloud/.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)

What is the related PR or file link(s)?

• This PR is translated from: N/A
• Other reference link(s):
  • Related issue: Add User Doc for TopRU tidb#67582
  • pingcap/docs-cn@4b8dd4f

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Code Review

This pull request introduces documentation for the TopRU feature in TiDB Cloud Clinic, including a new detailed guide and updates to the Table of Contents and main Clinic documentation. The feature allows users to identify high resource unit (RU) consumption by SQL statements and database users. The review feedback focuses on improving the documentation's clarity and adherence to the style guide by suggesting the use of active voice, removing redundant phrasing, and ensuring technical terms like TopN and sql_digest are properly formatted with backticks.