Document Merge Request Widget Extension Telemetry

What does this MR do and why?

This moves the README.md document from within the main source code (co-located with the widgets) into the documentation area.

As a result, it also leaves a few links to the new page so it can be discoverable while working on the code.

Related merge requests

• Related to Potential clarifying improvements to the widget... (!93323 - closed)
• Related to Move widget extension telemetry docs to the exi... (!93334 - merged)

MR acceptance checklist

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