chore: include link to documentation page in project urls

The project.urls are picked up and displayed (with matching icons, if
possible) by pypi.org. DataLad-next has its online documentation under
docs.datalad.org and it is pretty good. Given that we already include
a GitHub link in project.urls anyway (twice, in fact), it could be
better to point Documentation to the online docs, rather than the
GitHub README.

pyproject.toml

@@ -43,7 +43,7 @@ dependencies = [
 
 [project.urls]
 Homepage = "https://github.com/datalad/datalad-next"
-Documentation = "https://github.com/datalad/datalad-next#readme"
+Documentation = "https://docs.datalad.org/projects/next/en/latest/"
 Issues = "https://github.com/datalad/datalad-next/issues"
 Source = "https://github.com/datalad/datalad-next"
 Changelog = "https://github.com/datalad/datalad-next/blob/main/CHANGELOG.md"