Use latest/ directory for docs instead of stable/ (#3766)

See also
googleapis/google-cloud-python#3763

$ sed -i '' 's/googlecloudplatform.github.io\/google-cloud-python\/stable\//googlecloudplatform.github.io\/google-cloud-python\/latest\//g' **/*.rst

Author: tswast

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/latest/language/usage.html
 
 Quick Start
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