Load the analytics only in the main site (15.10 backport)

What does this MR do and why?

Backports !3897 (merged) to 15.10.

After this MR is merged, pipeline to follow and check that the image is rebuilt: https://gitlab.com/gitlab-org/gitlab-docs/-/pipelines?page=1&scope=all&ref=15.10

Merge request acceptance checklist

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