doc: Improvements to the Reproduce section

Various improvements to the Reproduce section:
- Merged the multiple statements related to kas having configuration
  files.
- Move Note from the middle of the introduction to its end.
- Change some of the wording to be more assertive.
- Moved reference for the customization to avoid having a new
  sub-section.

Issue-Id: SCM-5121
Signed-off-by: Filipe Rinaldi <filipe.rinaldi@arm.com>
Change-Id: I0810d86d0c81965ddebc10005e307bd85e407596