Problem

Many new contributors start at eitherhttps://matplotlib.org/devel/index.html orhttps://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 tohttps://matplotlib.org/devdocs/devel/index.html & add language about checking outhttps://matplotlib.org/ docs when developing
2. Add a link/obnoxious banner to the dev docs onhttps://matplotlib.org/devel/index.html