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/HACKING.rst b/HACKING.rst
index f8be0ad..95bcbb5 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -59,7 +59,7 @@
 `relevant plugin projects`_.
 
 .. _External Plugin Interface: https://specs.openstack.org/openstack/qa-specs/specs/tempest/implemented/tempest-external-plugin-interface.html
-.. _relevant plugin projects: https://docs.openstack.org/tempest/latest/plugin-registry.html#detected-plugins
+.. _relevant plugin projects: https://docs.openstack.org/tempest/latest/plugins/plugin-registry.html#detected-plugins
 
 Exception Handling
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