Create a landing page for website #30

Right now the front page of our website has announcements for different releases. An alternative would be to have a landing page (like the landing page for Astropy) that describes the project and points users on where to go, rather than having a series of announcements. (An advantage of that would be that we wouldn't have to create a new announcement for every release!) While we're doing this, we should also update our funding acknowledgements as per #28.

Thanks to @rocco8773 (I think) for bringing this up!