Add a "documentation" URL in the project metadata

Author: AA-Turner

pyproject.toml

@@ -9,6 +9,7 @@ description = "Python documentation generator"
 readme = "README.rst"
 urls.Changelog = "https://www.sphinx-doc.org/en/master/changes.html"
 urls.Code = "https://github.com/sphinx-doc/sphinx"
+urls.Documentation = "https://www.sphinx-doc.org/"
 urls.Download = "https://pypi.org/project/Sphinx/"
 urls.Homepage = "https://www.sphinx-doc.org/"
 urls."Issue tracker" = "https://github.com/sphinx-doc/sphinx/issues"