Add FHIR Resolution section to pkgdown documentation

- Introduced a new section in the pkgdown configuration for FHIR Resolution, detailing the translation of FHIR coded values to OMOP standard concepts.
- Added descriptions and contents for functions: `fhir_resolve`, `fhir_resolve_batch`, `fhir_resolve_codeable_concept`, and `omophub_fhir_url` to enhance documentation clarity and usability.

Author: alex-omophub

_pkgdown.yml

@@ -23,6 +23,17 @@ reference:
       - set_api_key
       - has_api_key
 
+  - title: FHIR Resolution
+    desc: >
+      Translate FHIR coded values to OMOP standard concepts and CDM
+      target tables via the FHIR Concept Resolver, plus helpers for
+      interop with external FHIR client libraries.
+    contents:
+      - fhir_resolve
+      - fhir_resolve_batch
+      - fhir_resolve_codeable_concept
+      - omophub_fhir_url
+
   - title: Response Classes
     desc: S3 classes for API responses
     contents: