readthedocs out-of-date, also should redirect to new docs location

[tswast]

Recent ReadTheDocs builds have been failing (due to requiring Python 3.6, which is no longer available on the ReadTheDocs environment) https://readthedocs.org/projects/pandas-gbq/builds/

That said, rather than post the full docs set there, we should (a) redirect to https://googleapis.dev/python/pandas-gbq/latest/index.html or (b) show a warning that https://googleapis.dev/python/pandas-gbq/latest/index.html is the official docs and make sure the "canonical" URL is pointing at the googleapis.dev version.