revert: restore correct pkgdown article paths

The reference-multidimensional and reference-validation vignettes live
in vignettes/ (not vignettes/articles/), so they should NOT have the
articles/ prefix. The previous "fix" was a false positive from the audit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JamesHWade

_pkgdown.yml

@@ -75,8 +75,8 @@ articles:
   desc: Information-oriented technical descriptions
   contents:
   - articles/reference-preprocessing
-  - articles/reference-multidimensional
-  - articles/reference-validation
+  - reference-multidimensional
+  - reference-validation
 - title: Explanation
   desc: Understanding-oriented discussion of concepts and architecture
   contents: explanation-internals