Update Contributing Environment section
Closes https://github.com/pandas-dev/pandas/issues/18041
Closes https://github.com/pandas-dev/pandas/issues/17544
Closes https://github.com/pandas-dev/pandas/issues/17747

Still need to

1. Maybe add a section on getting compilers
2. Update the windows environment section
3. More rewording on the docs

But I wanted to get this out there to check if people are OK with the general approach of

1. first conda environment.yaml file with all the build deps
2. second conda requirements file with all the optional deps
3. Instruct contributors to just install everything
4. A script to create requirements.txt files from the conda files for people using virtualenv / pip
5. Just focusing on python3

And if people are able to test this on their machine I'd be grateful. I've tested both conda and pip/venv on my mac.