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

首先定位 landing page 和 repository 的 Quarto 配置，包括其主题设置。添加一个包含三列的页脚，分别用于赞助者、资助信息、徽标和贡献链接；当页脚在主页上显示，并包含所要求的支持者类别时，工作即告完成。