ecosystem landing page

[story645]

As part of implementing #71, it might be good to pull the external-resources link in https://matplotlib.org/devdocs/users/index.html and instead have the third-party link in the header link out to a landing page that itself links out to the resources and libraries page.

• con one more layer of indirection 3PP->libraries->library page, 3PP->resources->resources page
• pros:
  • resources is currently buried in users->guide->resources so linking cost is net zero
  • puts building education resources on equal footing w/ building downstream libraries
  • process to contribute to both will be the same (+/- keywords) & lets us lead w/ make your own (like in the readme, which more or less probably could serve as the landing page)