Adds documentation about combined HA and Authentication uses

[IvanVas]

Description

Discussion: https://confluentcommunity.slack.com/archives/C6UJNMY67/p1658922529620649 Fixes https://github.com/confluentinc/ksql/issues/8256

Testing done

Previewed the docs.

Reviewer checklist

• [x] Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• [x] Ensure relevant issues are linked (description should include text like "Fixes #")

[CLAassistant]

All committers have signed the CLA.

[IvanVas]

Thanks @IvanVas - this will hopefully help a lot of people going forward! I'll merge it after addressing Jim's copy edits

Done, thanks!