docs: fix sbd_devices documentation and examples

[tomjelinek]

Enhancement:
Fix documentation and examples regarding configuring SBD devices in fence_sbd agent.

Reason:
ha_cluster.sbd_devices variable is used in examples even though it isn't defined. In other example the variable is defined, but its content depends on the node the task is executed on, which is bad practice for configuring sbd.

Result:
Examples and documentation are updated to show a recommended way of configuring SBD devices.

Issue Tracker Tickets (Jira or BZ if any):
https://issues.redhat.com/browse/RHELDOCS-18172

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.50%. Comparing base (06093e4) to head (87a8aa9).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #224   +/-   ##
=======================================
  Coverage   68.50%   68.50%           
=======================================
  Files           3        3           
  Lines         181      181           
=======================================
  Hits          124      124           
  Misses         57       57           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.