Documentation for postgresql citus support

[lmsurpre]

This experimental support was added under https://github.com/IBM/FHIR/issues/3437

Now we should document what that consists of and how to use it. Probably at least 3 locations:

1. Updates to the schema design documentation at https://github.com/IBM/FHIR/blob/main/fhir-persistence-schema/docs/SchemaDesign.md
2. Updates to the schema tool usage guide at https://github.com/IBM/FHIR/blob/main/fhir-persistence-schema/docs/SchemaToolUsageGuide.md
3. Updates to the User's guide with what feature is and how to configure it
4. more?

[lmsurpre]

I now see that #3683 does include some updates to the SchemaToolUsageGuide (number 2 in the description), its possible that this part is done (or at least started)

[lmsurpre]

didn't see any mention in the user's guide

please also make sure that any known limitations of the citus implementation are included in the docs

[lmsurpre]

Citus now has its own section in the user's guide under 3.3.1.1 Supported databases.

It also mentions the search limitations of the current impl:

Currently, some FHIR search queries which follow references between resources such as _include, _revinclude and _has do not work. Support for these search queries may be included in a future update.

Future enhancements can come as necessary.