[sponsor efforts] Add a dedicated marketing-focused sponsorship landing page

Currently the [prospectus](https://www.djangoproject.com/sponsor/) contains a lot of different benefits. The goal of this page would be a few things:

* Create a template for a dedicated landing page, so that future pages can be built that focus on different areas of sponsorship (eg. `Spnosor a fellow`)
* Build a landing page that we can link the `Sponsor us >` to in the places where marketing-focused benefits are fulfilled (eg. the sponsor listing in the banner / sidebars)
* Have a place to test marketing messages that you can iterate on in a defined way. In particular where you can measure the number of people who fill out the form in a specific time period.

Start by reading the existing prospectus at djangoproject.com/sponsor/ and tracing the “Sponsor us >” links from the banner, sidebars, and sponsor listing. Identify the existing page and form entry points before defining the reusable landing-page template. Done means a marketing-focused sponsorship page is linkable, supports measurable form submissions, and provides a template for future sponsorship pages.

Written by the indexing model from the issue text.