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/.gitignore b/.gitignore
index 9767e52..8b6222e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -31,7 +31,7 @@
 !.coveragerc
 cover/
 doc/source/_static/tempest.conf.sample
-doc/source/plugin-registry.rst
+doc/source/plugins/plugin-registry.rst
 
 # Files created by releasenotes build
 releasenotes/build