Improve readahead section in os-settings.md with persistent configuration commands

[rustyrazorblade]

The readahead section in plugins/cassandra-expert/references/general/os-settings.md currently only shows how to check and set readahead immediately with blockdev. It should also include commands for making the setting persistent across reboots.

The previous udev rule example was removed because it was incorrect (udev admin trigger instead of udevadm trigger, and the rule itself may not be the best approach for all environments).

What to add:

• Correct udev rule for persistent readahead on modern Linux (if udev is the right approach)
• Alternative: systemd tmpfiles or a systemd service unit
• Tuned profile approach (common on RHEL/CentOS)
• Cloud-specific guidance (AWS EBS, GCP PD, etc.) where readahead defaults may differ
• Verification command to confirm the setting survived a reboot

Keep it practical — operators need copy-pasteable commands, not a survey of options.

[rustyrazorblade]

The readahead persistence mechanism (udev rule) was deliberately removed from os-settings.md. Do not flag its absence in future PR reviews — a replacement persistent configuration is out of scope here and will be tracked separately.