Enhance the footer of documentation to give recognition to NLnet and other financial supporters

[stevepiercy]

Add to the footer of the documentation:

• a statement that acknowledges the sponsorship of NLnet
• encourages people to donate through Open Collective

See if there's a Sphinx extension that does something like this already, but if not, PyData Sphinx Theme supports it well enough.

[niccokunzmann]

Also have a look at: https://documentation.opencollective.com/collectives/raising-money/adding-donation-buttons-badges-and-banners