update homepage, navbar, conf and add how to become a community partner text

[reshamas]

Reference

• Closes #79
• referencing text from AFME2 scikit-learn sprint here: https://afme2021rc.dataumbrella.org/community-partners

[OriolAbril]

Don't merge yet. It is not rendering correctly, and I will also add some general comments

[OriolAbril]

How does this look? I think it is ready to merge now: https://pymc-data-umbrella--88.org.readthedocs.build/en/88