Add sponsors section to FEniCS 2023 web page

[francesco-ballarin]

Will move out of draft once:

• the organizing committee agrees on the wording of the section,
• we add missing logos, especially Università di Cagliari, for which we are waiting for authorization.

[drew-parsons]

Support from both NumFOCUS and University of Cagliari is now confirmed.

[drew-parsons]

The PR web interface here doesn't show the images web-formatted. Hopefully it looks ok in the final html. I gave the UniCA svg file a height of 50px, not certain if that's optimal (or needed).

[francesco-ballarin]

I've built locally, this is good to merge.