doc: Update intro, components and reproduce

* Add description of codeline management.
* Update the Maintainer(s) list.
* Add description of u-boot patch that disables setting
  COUNTER_FREQUENCY.
* Use kas version 3.1.

Signed-off-by: Qi Feng <qi.feng@arm.com>
Issue-Id: SCM-4874
Change-Id: I15f6b6bdea1308f5bfb540f85ba4f7ac2af79547