Rewrite K8s telemetry guide for clarity and completeness (#679)

* Rewrite K8s telemetry guide for clarity and completeness

- Fix passive voice, reduce "you can" overuse, use active constructions
- Restructure around MCPTelemetryConfig as the primary config path;
  collapse deprecated inline config to a minimal example
- Move configuration details (endpoint format, service name, sampling
  rate, Prometheus access) into the shared config section
- Add authentication headers section with generic sensitiveHeaders example
- Add minimal OpenTelemetry Collector config example
- Replace Honeycomb and Grafana Cloud "coming soon" placeholders with
  real MCPTelemetryConfig examples (closes #485)
- Simplify Jaeger section to use native OTLP support directly
- Fix Mermaid diagram Prometheus arrow directionality
- Fix endpoint note referencing CLI flag instead of insecure: true
- Fix "open-source" to "open source" per word list

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Address Copilot review feedback

- Clarify samplingRate is a quoted string, not a numeric value
- Fix inconsistent collector endpoint hostname across examples
- Clarify Grafana Cloud Secret needs full Authorization header value

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix sentence fragment and missing article

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Dan Barr <6922515+danbarr@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: danbarr