Add index page for plugins documents

Now we have more documents for plugins on stable release,
support and their testing policy. Let's prepare a
index page for it will be easy for plugins maintainer.

Also renaming the stable support policy section to have a clear
name.

Adding redirect links for moved documents.

Change-Id: I17a9a6e0e88605a0ee5047678d6ed168c31396ec
diff --git a/tools/generate-tempest-plugins-list.sh b/tools/generate-tempest-plugins-list.sh
index 961cd09..33675ed 100755
--- a/tools/generate-tempest-plugins-list.sh
+++ b/tools/generate-tempest-plugins-list.sh
@@ -98,8 +98,8 @@
 if [[ -r doc/source/data/tempest-plugins-registry.footer ]]; then
     cat doc/source/data/tempest-plugins-registry.footer
 fi
-) > doc/source/plugin-registry.rst
+) > doc/source/plugins/plugin-registry.rst
 
 if [[ -n ${1} ]]; then
-    cp doc/source/plugin-registry.rst ${1}/doc/source/plugin-registry.rst
+    cp doc/source/plugins/plugin-registry.rst ${1}/doc/source/plugins/plugin-registry.rst
 fi