docs: update CITATION.cff with author name and current version

- Replace generic "diff-diff contributors" with Isaac Gerber
  using the CITATION.cff spec's given-names/family-names format
- Bump version from 2.7.1 to 3.0.1
- Update date-released to 2026-04-07 (v3.0.1 release date)

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

Author: igerber

CITATION.cff

@@ -3,10 +3,11 @@ title: "diff-diff: Difference-in-Differences Causal Inference for Python"
 message: "If you use this software, please cite it as below."
 type: software
 authors:
-  - name: "diff-diff contributors"
+  - given-names: Isaac
+    family-names: Gerber
 license: MIT
-version: "2.7.1"
-date-released: "2026-03-18"
+version: "3.0.1"
+date-released: "2026-04-07"
 url: "https://github.com/igerber/diff-diff"
 repository-code: "https://github.com/igerber/diff-diff"
 keywords: