Link to Product Analytics documentation on Admin settings

What does this MR do and why?

Related to #381353 (closed)

After merging Add initial docs and schema to PA docs (!103029 - merged), we can now link to this page from the Product analytics admin setting section.

Screenshots or screen recordings

Link on settings header

Docs on preview app

How to set up and validate locally

1. Make sure you are on GitLab Ultimate.
2. Enable the cube_api_proxy feature flag

   Feature.enable(:cube_api_proxy)

3. Go to your instance General admin settings at http://127.0.0.1:3000/admin/application_settings/general
4. See the added link in the Product analytics section header.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.