observability: improve/restructure admin documentation

Created by: slimsag

Today

In our docs (https://docs.sourcegraph.com/admin/observability) we have:

• Observability
  • Metrics / Metrics and dashboards
    • Guide to metrics
    • Guide to dashboards
  • Tracing
  • Alerting
    • Alerting: custom consumption
  • Logs
  • Health checks
  • Other tools

And the generator for Grafana and Prometheus dashboards is named observability at the root of the repository.

Proposed

I think instead this should instead be:

• Observability
  • Monitoring
    • Dashboards
    • Alerting
    • Health checks
  • Debugging
    • Tracing
    • Logging

With the generator for Grafana dashboards and Prometheus rules being named monitoring at the root of the repository.

The most important aspect here is that "Observability" includes two categories:

1. Monitoring: How you know when something is wrong (Grafana, alerting, health checks)
2. Debugging: How you debug when you know something is wrong

Note: We already have monitoring and debugging labels on GitHub for filing issues in these categories, which is nice.