Add cephcsi troubleshooting guide

Currently, we don't have a troubleshooting guide to help users to understand the issue in first step. we need to come up with a document which helps in debugging the issues with cephcsi

* Which pod logs to check
* Any helper commands to identify the issue(in case of operation already exists, what logs user need to check and which command he execute and where to execute to check running/hang command)
* Force unmap if the mountpoint not present to unblock delete operation
* Check mon network connectivity from provisioner and plugin pods
etc...

No file, test, or documentation entry point is named. Start by reviewing the existing cephcsi documentation and operational guidance, then map the provisioner and plugin pods, mon connectivity checks, operation-hang logs and commands, and force-unmap procedure. Done means a troubleshooting guide covers each listed scenario with clear commands, locations, and expected checks.

Written by the indexing model from the issue text.