[RazorDocs] Add config-gated search telemetry and docs metrics

[akrock]

Summary

Add config-gated telemetry for RazorDocs search and wayfinding so findability improvements can be measured instead of guessed.

Scope

• Instrument events for query submitted, no results, filters changed, result clicked, and helpfulness feedback.
• Make telemetry configurable so self-hosted docs can disable it cleanly.
• Use an event shape that avoids collecting more query data than necessary.
• Document how to review the metrics and use them to tune docs titles, summaries, aliases, and navigation.

Acceptance Criteria

• Telemetry can be enabled or disabled with configuration.
• Event hooks exist for queries, no results, refinements, click-throughs, and helpfulness signals.
• Privacy behavior and data shape are documented.
• Tests cover disabled behavior and event emission contracts where practical.