Bump version to v0.3.1 for release

Author: jakep-allenai

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+## [v0.3.1](https://github.com/allenai/olmocr/releases/tag/v0.3.1) - 2025-08-14
+
 ## [v0.3.0](https://github.com/allenai/olmocr/releases/tag/v0.3.0) - 2025-08-13
 
 ## [v0.2.3](https://github.com/allenai/olmocr/releases/tag/v0.2.3) - 2025-08-04