Docs: Replace links to '/stable/' with '/latest/'. (#5901)

* Replace links to '/stable/' with '/latest/'.

* DRY out duplicated 'README.rst' vs. 'docs/index.rst'.

* Include websecurityscanner in docs.

Toward #5894.

packages/google-cloud-trace/docs/index.rst

@@ -10,7 +10,7 @@ applications can be provided using this API.
 
 .. _Alpha: https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/README.rst
 .. _Stackdriver Trace API: https://cloud.google.com/trace
-.. _Client Library Documentation: https://googlecloudplatform.github.io/google-cloud-python/stable/trace/usage.html
+.. _Client Library Documentation: https://googlecloudplatform.github.io/google-cloud-python/latest/trace/usage.html
 .. _Product Documentation:  https://cloud.google.com/trace
 
 Quick Start
@@ -26,7 +26,7 @@ In order to use this library, you first need to go through the following steps:
 .. _Select or create a Cloud Platform project.: https://console.cloud.google.com/project
 .. _Enable billing for your project.: https://cloud.google.com/billing/docs/how-to/modify-project#enable_billing_for_a_project
 .. _Enable the Stackdriver Trace API.:  https://cloud.google.com/trace
-.. _Setup Authentication.: https://googlecloudplatform.github.io/google-cloud-python/stable/core/auth.html
+.. _Setup Authentication.: https://googlecloudplatform.github.io/google-cloud-python/latest/core/auth.html
 
 Installation
 ~~~~~~~~~~~~
@@ -99,4 +99,4 @@ Api Reference
     :maxdepth: 2
 
     gapic/v2/api
-    gapic/v2/types
+    gapic/v2/types