Add new query=epaSearch

[bertelsmann-hack080]

This pull request updates the FHIR VZD search documentation to introduce a new, optimized search query for ePA organization lookups and to clarify some existing documentation. The main focus is on providing a streamlined way for ePA frontend applications to search for healthcare services and organizations using a new custom query, reducing complexity and improving performance.

New ePA organization search documentation:

• Added a detailed section describing the new _query=epaSearch custom FHIR query for searching healthcare services in the ePA context, including supported parameters, behavior, and example requests. This query allows for efficient filtering by ePA version, ProfessionOID, DiGA inclusion/exclusion, and organization identifier, and returns associated resources.

Documentation improvements and clarifications:

• Clarified the note about searching in all identifier values, correcting a minor typo and improving the explanation for the identifier search parameter.