clinic: add Top RU user guide for TiDB Cloud and update navigation (ref pingcap/tidb#67582)

[zimulala]

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

This PR adds a new Top RU user guide under the PingCAP Clinic section for TiDB Cloud usage, instead of placing it under self-managed TiDB Dashboard docs.

Key changes:

• Add a new page: clinic/top-ru.md
• Add Top RU image assets used by the new page:
  • media/dashboard/top-ru-instance-view.png
  • media/dashboard/top-ru-instance-detail.png
  • media/dashboard/top-ru-db-user-view.png
  • media/dashboard/top-ru-db-user-detail.png
  • media/dashboard/top-ru-vs-ru-trend.png
  • media/dashboard/top-ru-vs-qps-trend.png
• Update navigation:
  • Add Top RU 页面 entry under PingCAP Clinic 诊断服务
  • Remove Top RU 页面 entry from TiDB Dashboard

Also, top-ru-instance-view.png is cleaned to remove unrelated UI overlay icon.

Related issue: pingcap/tidb#67582

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 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)

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

• This PR is translated from: N/A
• Other reference link(s):
  • TiDB issue: Add User Doc for TopRU tidb#67582
  • Top SQL reference page: https://docs.pingcap.com/zh/tidb/stable/top-sql/
  • Internal material: https://pingcap.feishu.cn/wiki/E3yQwhQ4eisLMqkPiQNcQMZAnOb

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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