Fix Broken Links with Read the Docs

[grant]

There seems to be links in the docs that link to DevSite.

Site: https://google-auth-oauthlib.readthedocs.io/en/latest/reference/google_auth_oauthlib.flow.html
Text: "Installed Application Authorization Flow"

links to https://developers.google.com/api-client-library/python/auth/installed-app

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This should link to GitHub instead of DevSite:
https://github.com/googleapis/google-api-python-client/blob/master/docs/oauth-installed.md