Clarify administrative case semantics and add CHCoreEpisodeOfCare profile

[luisfabib]

This PR clarifies how the Swiss administrative case concept (Fall) is represented and documented in CH Core. Closes #431

The branch introduces a logical model for CHAdministrativeCase, adds a CHCoreEpisodeOfCare profile with the same CaseNumber semantics already used on CHCoreEncounter, and improves the surrounding documentation so the distinction between a clinical resource and an administrative case is explicit.

Why

In Swiss practice, the administrative case number is relevant for administrative and billing workflows, but it is not identical to a FHIR Encounter or EpisodeOfCare (see #431).

This PR makes that distinction clearer by:

• Documenting the administrative case as a logical model rather than an exchange resource
• Showing where corresponding case information can be found in existing CH Core artifacts

The goal is not to introduce a new exchange resource for administrative cases. The goal is to give implementers a clearer conceptual model for a business concept that already exists in Swiss practice, and to document how that concept can be represented, partially, through existing CH Core FHIR resources.

Added

• Add a new logical model CHAdministrativeCase
• Mapped both CHCoreEncounter and CHCoreEpisodeOfCare to the new logical model
• Profiled CHCoreEpisodeOfCare with an identifier:CaseNumber slice to hold case information

Changed

• Renamed the Encounter.identifier slice from VisitNumber to CaseNumber
• Aligned the Encounter example instances to the introduced changes
• Clarified and homogenized the short descriptions and translations of the different BFS-oriented elements in CHCoreEncounter

Points for discussion

1. Should CH Core include a logical model for the administrative case at all?
   This PR assumes there is value in documenting the concept explicitly, even if it is not represented as a standalone FHIR resource.

2. Are Encounter and EpisodeOfCare sufficient to carry case information or are there others?
   The branch proposes that both can carry administrative case information, depending on context, and that neither should be treated as identical to the administrative case itself.

3. Is CaseNumber the right name and semantic scope?
   The PR uses CaseNumber instead of VisitNumber to better reflect the Swiss administrative concept and to be applicable to other resources such as EpisodeOfCare.

4. Is the logical model too minimal, or already too ambitious?
   The current field set is intentionally small. The group should decide whether CH Core should stop at identifier plus basic dates, or whether the model should also cover additional administrative dimensions.

5. Does the documentation describe the Swiss context accurately enough?
   In particular, the group should review whether the current wording around inpatient/outpatient usage is helpful or whether it risks oversimplifying real Swiss workflows.