Fix broken virtual environment guide link on contributing page

[RDaxini]

Describe the bug
On the contributing page, the link to the astropy page on virtual environments does not work.

To Reproduce
Steps to reproduce the behavior:

1. Go to the Basics section
2. Click on astropy guide
3. See error

Expected behavior
I think the correct URL is https://astropy-astrofrog.readthedocs.io/en/latest/development/workflow/virtual_pythons.html but could someone confirm?
This link to the pvlib user guide section on setting up a virtual environment could also be integrated into the same sentence.

Versions:
Checked 10.5, 11.0 (stable), and 11.1 (latest) --- link not working