WEB: Content for the footer
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
- ...
