Set up donation buttons and supporters sections on mybinder.org

[choldgraf]

As a part of jupyterhub/team-compass#430 we decided to make the following pages to mybinder.org:

On the home page and the loading page

• Add a "support mybinder.org" call-to-action button. It points to https://numfocus.salsalabs.org/donate-to-binder

On the home page

Add a "supporters of mybinder.org" section below the fold. This would be modeled after the jupyter.org sponsors page. It would have three sections:

• mybinder.org federation members - entities that deploy, fund, and staff their own BinderHub in the federation.
• mybinder.org partner institutions - entities that employ individuals that operate any hubs in the mybinder.org federation (if not full federation members).
• mybinder.org supporters - entities that provide significant financial resources or credits for any hubs in the mybinder.org federation ("significant" is defined as anything greater than $10,000 annually).

It would also link to a "support mybinder.org" page in the Binder docs that had more information about how Binder's donations work.

[choldgraf]

I had a quick thought about using docs.mybinder.org to list this instead of mybinder.org directly, would love any feedback about that in this issue:

• Add page on our "supporters" and donation infrastructure mybinder.org-user-guide#257