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.

Author: tseaver

docs/index.rst

@@ -8,7 +8,7 @@ Python Client for Google Cloud Firestore API (`Alpha`_)
 
 .. _Alpha: https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/README.rst
 .. _Google Cloud Firestore API: https://cloud.google.com/firestore
-.. _Client Library Documentation: https://googlecloudplatform.github.io/google-cloud-python/stable/firestore/usage.html
+.. _Client Library Documentation: https://googlecloudplatform.github.io/google-cloud-python/latest/firestore/usage.html
 .. _Product Documentation:  https://cloud.google.com/firestore
 
 Quick Start
@@ -24,7 +24,7 @@ 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 Google Cloud Firestore API.:  https://cloud.google.com/firestore
-.. _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
 ~~~~~~~~~~~~
@@ -80,4 +80,4 @@ Api Reference
     :maxdepth: 2
 
     gapic/v1beta1/api
-    gapic/v1beta1/types
+    gapic/v1beta1/types