Problem

Many new contributors start at either https://matplotlib.org/devel/index.html or https://matplotlib.org/users/installing.html and often end up installing the latest release rather than the dev release from master.

Suggested Improvement

these were both proposed on today's GSOD call:

1. redirect the contribute button on the homepage to https://matplotlib.org/devdocs/devel/index.html & add language about checking out https://matplotlib.org/ docs when developing
2. Add a link/obnoxious banner to the dev docs on https://matplotlib.org/devel/index.html