Add Documentation link to project.urls for PyPI (#435)

Also could go [directly to the quickstart][1]?

Ref:
https://packaging.python.org/en/latest/specifications/well-known-project-urls/#well-known-labels

[1]: https://protovalidate.com/quickstart-python/

Author: stefanvanburen

pyproject.toml

@@ -43,6 +43,7 @@ dependencies = [
 Homepage = "https://github.com/bufbuild/protovalidate-python"
 Source = "https://github.com/bufbuild/protovalidate-python"
 Issues = "https://github.com/bufbuild/protovalidate-python/issues"
+Documentation = "https://protovalidate.com"
 
 [dependency-groups]
 dev = [