Document access and queries for extension telemetry

## Summary
Publish an internal short guide for Microsoft extension authors to access extension telemetry and query recent events.

## Scope
- Add an `aka.ms` link (or equivalent stable short link) to the relevant extension telemetry documentation (possibly in -pr?)
- Explain how authors obtain access to telemetry data.
- Include an example query showing the last N occurrences of the `demo.telemetry.reported` event from the demo extension
- Keep the query aligned with the actual Application Insights table and current event/field names.
- Print the aka.ms link from the demo extension when telemetry was successfully collected

## Reference
Follow-up from [PR #9174 review comment](https://github.com/Azure/azure-dev/pull/9174#discussion_r3754638548).