doc: Add application documentation

Add a new section for application, and move the exist content from
comonents into it.

Issue-Id: SCM-5624
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Qi Feng <qi.feng@arm.com>
Change-Id: I2160cd81bbf112f060ebebc5e33a4bab616e1ae5