Docs: Redirect gh-pages to googleapis.dev.

[busunkim96]

Add redirects for stable/ and latest/. Anyone visiting the docs at googleapis.github.io or readthedocs will be redirected to googleapis.dev.

• If the page is not for a specific API, redirect to the google-cloud-python README.
  https://github.com/googleapis/google-cloud-python#google-cloud-python-client
• If the page is for an API, try to redirect to the corresponding api page.
  https://googleapis.github.io/google-cloud-python/latest/redis/index.html -> https://googleapis.dev/python/redis/latest/gapic/v1beta1/api.html
• If there is no corresponding page at googleapis.dev, redirect to the {api}/latest
  https://google-cloud-python.readthedocs.io/en/stable/monitoring/resource.html -> https://googleapis.dev/python/monitoring/latest

Should be merged after #9085

[tswast]

From Kokoro and GitHub, it appears there are merge conflicts to resolve first.