Migrate docs to googleapis.dev

[JustinBeckwith]

I noticed this repository is still sitting on readthedocs. We need to move this over to googleapis.dev to be consistent with the rest of the libraries we publish.

[busunkim96]

The docs have been publishing to both places for a few versions now: https://googleapis.dev/python/google-auth/latest/index.html. The work remaining is to get the readthedocs docs to redirect to googleapis.dev.

[tswast]

Note: a search for "google auth python" still shows readthedocs as the top link:

[parthea]

Closing as obsolete