carp: Create advanced troublshooting guide

[Monviech]

Important notices

Before you add a new report, we ask you kindly to acknowledge the following:

• I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• I have searched the existing issues and I am convinced that mine is new.

Is your feature request related to a problem? Please describe.

Carp can be a challenge to troubleshoot, having a docs article that would explain some common and advanced scenario could help us and others.

Describe the solution you like

• Common and intermediate errors with CARP (e.g. multicast issues, split-brain scenario, flapping interfaces, TX/RX errors etc...)
• Which log files are important
• Which patterns in log files to look out for, common CARP errors and other related errors
• CARP debug logging
• important sysctl values and how to reset them
• viewing and comparing the VIP hashes
• Checking CARP configuration for errors

It does not have to be perfect, it should give an overview of a troubleshooting workflow with the most common errors (on shell). More specific errors can be added later to improve this document.

I want the document to be less like a technical documentation and more like a manual/how-to