Add sponsorship section

[valencik]

We should have a page that thanks our current sponsors, displays their logos, and provides a link to the open collective site.
(https://opencollective.com/typelevel)

[armanbilge]

Here's how we do it on the Cats landing page.

https://github.com/typelevel/cats/blob/c2b8c1e9b287474722437dd94b05ffc1dc38025a/docs/index.md?plain=1#L32-L66

https://github.com/armanbilge/cats/blob/c2b8c1e9b287474722437dd94b05ffc1dc38025a/docs/js/sponsors.js