Clarify and improve STONITH requirement

[tahliar]

PR creator: Description

I've addressed two JIRA issues in this PR:

DOCTEAM-144

• Clarified the STONITH item in the NFS quick start guide. This section is a usage scenario rather than a list of requirements, so not an appropriate place to go into a lot of detail.
• Added more detail to the STONITH requirement in the Installation quick start guide.

DOCTEAM-107

• Added an appendix with simple procedures to add iSCSI storage for use with SBD. I've tried to clearly mark it as just a simple option for testing purposes, but I think it's really useful to include as an easy way to get SBD for testing out an HA cluster, even if you know nothing about shared storage.

PR creator: Are there any relevant issues/feature requests?

• bsc#1158162
• jsc#DOCTEAM-144
• jsc#DOCTEAM-107

PR creator: Which product versions do the changes apply to?

When opening a PR, check all versions of the documentation that your PR applies to.

• SLE-HA 15
  • 15 next (current main, no backport necessary)
  • 15 SP4
  • 15 SP3
  • 15 SP2
  • 15 SP1
• SLE-HA 12
  • 12 SP5
  • 12 SP4

PR reviewer only: Have all backports been applied?

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When opening a PR, do not set the following check box.

• all necessary backports are done

[dariavladykina]

Hi, just minor corrections referring to this update, otherwise LGTM. Thanks!