Enhance Agent best practices throughout the documentation

Enhance Agent best practices throughout the documentation

- Highlight the Icinga agent
- Prefer SSH as fallback and add more detailed setup instructions
- Improve SNMP and add traps right after
- Explain nscp and wmi with URLs
- Drop discouraged nrpe instructions (checks and PNP customizations)
- Update Dependency examples with the Icinga Agent

This sources from recent discussions on community.icinga.com
and follows the updates with the distributed monitoring chapter for 2.11.