doc: Update links to external documentation with new versions

This patch updates:
  * links to the Yocto Project documentation to point to 3.4.3
  * links to the xl documentation to point to 4.15
  * links and documentation for using kas to point to 3.0.2
  * links to kernel reference configuration to use the yocto-5.14 versions

Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Ib518a55596c7b7532341c1ce6adedf9e13025528