Fixing references to "dead" docs links. (#3631)

* Fixing references to "dead" docs links.

Done via:

$ git grep -l 'google-cloud-auth.html' | \
> xargs sed -i s/'google-cloud-auth.html'/'core\/auth.html'/g
$ git grep -l 'http\:\/\/google-cloud-python.readthedocs.io' | \
> xargs sed -i s/'http\:\/\/google-cloud-python.readthedocs.io'/\
> 'https\:\/\/google-cloud-python.readthedocs.io'/g

Fixes #3531.

* Fixing up other docs that were moved in #3459.

packages/google-cloud-language/README.rst

@@ -9,7 +9,7 @@ Python Client for Google Cloud Natural Language
 
 -  `Documentation`_
 
-.. _Documentation: https://googlecloudplatform.github.io/google-cloud-python/stable/language-usage.html
+.. _Documentation: https://googlecloudplatform.github.io/google-cloud-python/stable/language/usage.html
 
 Quick Start
 -----------
@@ -28,7 +28,7 @@ possible. Check out the `Authentication section`_ in our documentation to
 learn more. You may also find the `authentication document`_ shared by all
 the ``google-cloud-*`` libraries to be helpful.
 
-.. _Authentication section: http://google-cloud-python.readthedocs.io/en/latest/google-cloud-auth.html
+.. _Authentication section: https://google-cloud-python.readthedocs.io/en/latest/core/auth.html
 .. _authentication document: https://github.com/GoogleCloudPlatform/gcloud-common/tree/master/authentication
 
 Using the API