Update virtual environment guide URL on contributing page (#2200)

* Update contributing.rst

* add scientific-python.org URL

* update hyperlinked text

* link between user guide sections

* Update contributing.rst

docs/sphinx/source/contributing.rst

@@ -68,12 +68,14 @@ into detail on each of these steps. Also see GitHub's `Set Up Git
 Requests <https://help.github.com/articles/using-pull-requests/>`_.
 
 We strongly recommend using virtual environments for development.
-Virtual environments make it trivial to switch between different
-versions of software. This `astropy guide
-<http://astropy.readthedocs.org/en/latest/development/workflow/
-virtual_pythons.html>`_ is a good reference for virtual environments. If
-this is your first pull request, don't worry about using a virtual
-environment.
+Virtual environments make it easier to switch between different
+versions of software. This `scientific-python.org guide
+<https://learn.scientific-python.org/development/tutorials/dev-environment/>`_
+is a good reference for virtual environments. The pvlib-python `installation
+user guide <https://pvlib-python.readthedocs.io/en/stable/user_guide/
+installation.html#set-up-a-virtual-environment>`_ also provides instructions on
+setting up a virtual environment. If this is your first pull request, don't
+worry about using a virtual environment.
 
 You must include documentation and unit tests for any new or improved
 code. We can provide help and advice on this after you start the pull