Add footer with funding information

@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

まず、ランディングページと、テーマ設定を含むリポジトリの Quarto 設定を見つけます。スポンサー、資金提供情報、ロゴ、貢献用リンクのための3列のフッターを追加します。フッターが、要求された支援者カテゴリとともにホームページに表示されれば作業完了です。