Documentation Guide: Set `RLIMIT_NOFILE` for Scylla pods

[mflendrich]

#2160 implemented a mechanism to set the max open files rlimit (RLIMIT_NOFILE) for a Scylla node via the fs.nr_open sysctl.

#2457 is implementing a way to set the sysctls on the node level, including (but not limited to) fs.nr_open.

The scope of this issue is to get users familiar with this mechanism, and to explain how to set RLIMIT_NOFILE for Scylla nodes under Operator.

Acceptance Criteria

• A guide explaining how to set the max open files limit exists.
• Stretch: make some increment towards the docs structure outlined in Structure the documentation for the reader #2787. This should fall under Production-grade Deployment -> Custom Scenarios