The 'all-plugin' tox environment was deprecated by this patch [1].
Instead of the 'all-plugin' it is recommended to use the 'all' tox
environment.
This patch removes any reference to 'all-plugin' tox environment and
updates the documentation so that the installation steps work with
the 'all' tox venv.
[1] https://review.opendev.org/c/openstack/tempest/+/543974
Related-Bug: 1862240
Change-Id: I0a86d9cf33bff6576a6337967d6b663ab7d9f4a6
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