Docs: Remove instance-level analytics page

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

The page Instance-level analytics is not necessary because it doesn't document a feature, it only links to DevOps reports and usage trends.

Proposal

• Remove the page Instance-level analytics.
• Remove the entry instance-level analytics from the analytics features table.
• Remove instance-level analytics from the left nav. gitlab-docs!5295 (merged)

Who can address the issue

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