doc,armv8r64-extras/kas: Pin refspecs for v5.0

* Introduce the "meta-armv8r64-extras/kas/release/v5.0.yml" and pin
  the dependent layers' refspec in it
* Have the "meta-armv8r64-extras/kas/include/base.yml" including the
  file above
* Update the "documentation/user-guide/reproduce.rst" to

    git clone https://gitlab.arm.com/automotive-and-industrial/v8r64

 \
        -b v5.0

Issue-Id: SCM-5610
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: Id1c65d15e7670765ba78279ff2c698f893c189fb