docs: document special exception for machine type deprecation & removal

docs: document special exception for machine type deprecation & removal

This extends the deprecation policy to indicate that versioned machine
types will be marked deprecated after 3 years, and then subject to
removal after a further 3 years has passed.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240620165742.1711389-15-berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>