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-python-speech/README.rst

@@ -26,7 +26,7 @@ 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
@@ -41,7 +41,7 @@ and receive a text transcription from the Cloud Speech API service.
 See the ``google-cloud-python`` API `speech documentation`_ to learn how to
 connect to the Google Cloud Speech API using this Client Library.
 
-.. _speech documentation: https://googlecloudplatform.github.io/google-cloud-python/stable/speech-usage.html
+.. _speech documentation: https://googlecloudplatform.github.io/google-cloud-python/stable/speech/usage.html
 .. |pypi| image:: https://img.shields.io/pypi/v/google-cloud-speech.svg
    :target: https://pypi.python.org/pypi/google-cloud-speech
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-speech.svg