Fix pkgdown: add popDat datasets to reference index

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bschilder

_pkgdown.yml

@@ -96,3 +96,8 @@ reference:
     desc: Minor allele frequency retrieval.
     contents:
       - get_MAF_UKB
+
+  - title: Bundled data
+    contents:
+      - popDat_1KGphase1
+      - popDat_1KGphase3