docs: reflect Glean-derived nature of popularities.json

[LeoMcA]

Summary

See also:

• chore(popularities): use Glean popularities #8808
• remove checked in popularities.json content#7551
• Make popularities.json dynamic instead of checked into git #4286
• Make popularities.json dynamic instead of checked into git #4217

Problem

We didn't update the documentation around popularities when making the above changes, nor remove duped commands from the build.

Solution

Update the docs, remove the duped commands.

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How did you test this change?

No testing done

[caugner]

LGTM, thanks! 🙌