docs(drain cleaner): clarify configuration requirements or webhook resource

[PaulRMellor]

Documentation

Drain Cleaner doc update
Clarifies that for most users, no configuration is needed for the webhook resource
Mentions requirement for custom certificates

Checklist

Please go through this checklist and make sure all applicable tasks have been done

• Write tests
• Make sure all tests pass
• Update documentation
• Check RBAC rights for Kubernetes / OpenShift roles
• Try your changes from Pod inside your Kubernetes and OpenShift cluster, not just locally
• Reference relevant issue(s) and close them after merging
• Update CHANGELOG.md
• Supply screenshots for visual changes, such as Grafana dashboards

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.93%. Comparing base (56db592) to head (f95bde6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #12046      +/-   ##
============================================
+ Coverage     74.90%   74.93%   +0.02%     
- Complexity     6453     6456       +3     
============================================
  Files           343      343              
  Lines         24336    24336              
  Branches       3207     3207              
============================================
+ Hits          18230    18237       +7     
+ Misses         4832     4827       -5     
+ Partials       1274     1272       -2     

see 3 files with indirect coverage changes

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