cassini-[doc,config,distro]: Cassini 0.9.0 release

For the v0.9.0 release,:
  * Sets DISTRO_VERSION to v0.9.0
  * Pins all dependent layers
  * Updates the documentation mentioning the v0.9.0 in the relevant
sections

Changelog: other

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>