Fix README rst links [(googleapis#962)](GoogleCloudPlatform/python-docs-samples#962)

* Fix README rst links

* Update all READMEs

Author: waprin

samples/hello/README.rst

@@ -108,11 +108,11 @@ This sample uses the `Google Cloud Client Library for Python`_.
 You can read the documentation for more details on API usage and use GitHub
 to `browse the source`_ and  `report issues`_.
 
-.. Google Cloud Client Library for Python:
+.. _Google Cloud Client Library for Python:
     https://googlecloudplatform.github.io/google-cloud-python/
-.. browse the source:
+.. _browse the source:
     https://github.com/GoogleCloudPlatform/google-cloud-python
-.. report issues:
+.. _report issues:
     https://github.com/GoogleCloudPlatform/google-cloud-python/issues