Add footer with funding information

[WardBrian]

@bob-carpenter:

I think we should add a list of sponsors to the bottom of the home page. I like the way PyMC did this, but would suggest three columns for us. I'm happy to help with the formatting.

@mitzimorris:

I think a footer on the landing page with a set of logos of all the folks who've supported Stan would be good, and a good place to put a button for contributions and ways to fund Stan past funding: feds: DoE, NSF, NIH, ONR foundations: Simons Foundation, Sloan, Schmidt Futures, companies: Meta, Google, Metrum, Jumping Rivers

[bob-carpenter]

I pinged the forums to try to collect all that we need. We should also include YouGov and Novartis for company support. And IES (ed. dept) for federal funding for me and Andrew. I don't know if we want to include all the StanCon sponsors as Stan sponsors. We also want to brand Flatiron Institute, not Simons Foundation (very confusing, since copyright goes to SF).

Here's a link to the forum post:

https://discourse.mc-stan.org/t/stan-funding-sources-for-web-page/38496/1