infrastructure-monitoring-and-testing: Add document

Discuss challenges and approaches wrt. infrastructure maintenance.

Rendered PDF: https://gitlab.apertis.org/docs/apertis-website/-/jobs/457303/artifacts/file/public/designs/infrastructure-monitoring-and-testing/infrastructure-monitoring-and-testing.pdf

️ https://www.apertis.org/architecture/infrastructure-monitoring-and-testing/