Switch GitHub pages from "branch" source to "actions"/API source

[mfisher87]

I just can't stand the gh-pages branch. Any preferences? It's an easy migration.

[asteiker]

@mfisher87 We see that #325 was closed, but gh-pages branch still exists, and in the deployment settings. Do we just need to close the branch to call this complete?

[mfisher87]

We can close this without making any changes. We dropped GitHub Pages for docs (effectively) and reset the gh-pages branch to only include a redirect to ReadTheDocs. Can be tested at https://nsidc.github.io/earthaccess. Source at https://github.com/nsidc/earthaccess/blob/gh-pages/index.html