Consolidate metrics & Sonar documentation

[ArBridgeman]

Summary

Since #785 , we only use Sonar for collecting metrics. It'd be nice if our documentation were consolidated and improved some to reflect that.

Relevant pages:

• https://exasol.github.io/python-toolbox/main/user_guide/features/metrics/collecting_metrics.html
• https://exasol.github.io/python-toolbox/main/user_guide/features/metrics/sonar.html
• https://exasol.github.io/python-toolbox/main/user_guide/features/metrics/ignore_findings.html