Add Documentation for FHIR Connection Configuration

[schwzr]

[schwzr]

see datasharingframework/dsf#377

[hhund]

Examples see: https://github.com/datasharingframework/dsf/blob/develop_2/dsf-bpe/dsf-bpe-server/src/test/java/dev/dsf/bpe/config/FhirClientConfigYamlReaderTest.java#L69-L147