Document telemetry usage and configuration in DEVELOPER_GUIDE.md

## Summary
`DEVELOPER_GUIDE.md` covers environment setup, testing, and contributing workflow, but does
not describe Solr Orbit's telemetry system.

## What's needed
Add a section to `DEVELOPER_GUIDE.md` that covers:

- The available telemetry devices and what each measures (JVM, node stats, collection stats,
query stats, indexing stats, cache stats, segment stats, shard stats, etc.)
- Which devices are always-on vs. opt-in
- Which devices require a provisioned (managed) Solr vs. an external cluster
- How to enable/disable devices via the `telemetry.devices` parameter in a track/workload
- How to extend or add new telemetry devices

## Why
New contributors and users are left to read the source in `osbenchmark/telemetry.py` to
understand what telemetry is collected — this should be documented at the guide level.