[sponsor efforts] Add pricing block for sponsorships

The current Django [prospectus](https://www.djangoproject.com/sponsor/) is good as a reference, but it's overwhelming for new sponsors. It would be really useful to have a "pricing page" summary for the sponsors.

This would help people get a quick / high-level understanding of which sponsorship is right for them, which they can dive into after that.

An example from [Rust](https://rustfoundation.org/get-involved/):

cc @jefftriplett @nessita

Start by reviewing the current Django prospectus at djangoproject.com/sponsor/ and the Rust Foundation example linked in the issue. Identify the sponsorship information that needs a high-level pricing summary, then add a pricing block or page that helps new sponsors choose an appropriate sponsorship level before reading the full prospectus.

Written by the indexing model from the issue text.