WEB: Content for the footer

[datapythonista]

What do we want in the new website footer? Will be visible in all the pages (including the documentation I guess)?

Ideas:

• Copyright note
• NumFOCUS note (see Dask website)
• Social media links (GitHub and Twitter I think)
• Back to top link
• Note about sponsors (like the providers of hosting and the design, see Django website)
• Project license
• Trademark note (see Spark website)
• Terms and conditions, privacy note (see Tensorflow page)
• Note link to donate
• ...