DOC: Add instructions how to activate virtual env under windows with pip
Contributing.rst now correctly describes how to setup dev environment
under Windows when using pip and not conda.

Closes #29112

- [x] closes #29112 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
