Lots of small changes to the website and sponsorship

[ericholscher]

I did a quick cleanup of the front page and website sponsorship, which is where this started. It mostly just cleans things up, but it did change:

• Sidebar sponsorship is now just monthly
• Footer sponsorship is yearly
• I also added a contact us page, because that seems useful..

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:books: Documentation preview :books:: https://writethedocs-www--1891.org.readthedocs.build/