Remove the duplicated/outdated 'activation' ping docs

[Dexterp37]

This additionally overhauls the Fenix telemetry docs to point at the Glean autogenerated docs, to prevent documentation getting outdated again.

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

After merge

• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

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[AaronMT]

The ui-test-x86-debug error for the test indicated should be fixed in a more recent rebase.

[Dexterp37]

The ui-test-x86-debug error for the test indicated should be fixed in a more recent rebase.

Yup, being a doc-only change I believe we could also just merge once review :-)

[liuche]

doc updates, lgtm!