Add missing metrics pages to openvox-server nav

[miharp]

Problem

The following pages exist in the openvox-server collection but have no nav entry and are not linked from any other page in the collection:

• puppet_server_metrics.html — Monitoring OpenVox Server metrics
• http_client_metrics.html — HTTP client metrics (sub-page of the above)
• puppet_server_metrics_performance.html — Applying metrics to improve performance

These were previously only reachable via the legacy index.markdown ToC. They were absent from the nav before that ToC was replaced in #152, so this is a pre-existing gap rather than a regression.

Goal

Add these pages to the nav under "Tuning, troubleshooting, and known issues", and review their content for Puppet branding while there.

[miharp]

Update

Branch feat/fix-orphaned-metrics-157 is ready, waiting on #152 to merge before opening a PR.

What's in the branch

• All three pages fully modernized (OpenVox Server branding, OpenVoxDB in prose, PE block and dead links removed, "compile masters" → "compilers", recommended FOSS dashboard updated to puppet-operational-dashboards to match current Puppet Core 8 upstream)
• New "Metrics" nav section (replacing the old "Metrics API endpoints" section) that combines the three previously orphaned conceptual pages with the existing v1/v2 metrics API endpoint pages — modeled on how Puppet's docs group metrics content under a single section

Nav structure

- text: Metrics
  items:
  - text: Monitoring OpenVox Server metrics
  - text: HTTP client metrics
  - text: Applying metrics to improve performance
  - text: v1 metrics API
  - text: v2 (Jolokia) metrics API