doc: Restructure 'Introduction' and 'Reproduce' section

As part of this commit:
- Pair 'Use-Cases' in 'Introduction' to 'Reproduce' steps
- Make 'Use-Cases' section more user-friendly
- Specify how to exit from a FVP run.

Signed-off-by: Vineeth Raveendran <vineeth.raveendran@arm.com>