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/doc/source/index.rst b/doc/source/index.rst
index b590380..f878888 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -87,8 +87,7 @@
 .. toctree::
    :maxdepth: 2
 
-   plugin
-   plugin-registry
+   plugins/index
 
 Tempest & Plugins Compatible Version Policy
 -------------------------------------------
@@ -98,6 +97,14 @@
 
    tempest_and_plugins_compatible_version_policy
 
+Stable Branch Support Policy
+----------------------------
+
+.. toctree::
+   :maxdepth: 2
+
+   stable_branch_support_policy
+
 Stable Branch Testing Policy
 ----------------------------
 
@@ -114,14 +121,6 @@
 
    library
 
-Support Policy
---------------
-
-.. toctree::
-   :maxdepth: 2
-
-   stable_branch_support_policy
-
 Search
 ======