doc : Add user guide for the Flame graph view

doc : Add user guide for the Flame graph view

This shows what a cpu flame graph is, how to use the view and
an example workflow.

Change-Id: I7f6f6ccf1682b5396ef9ec24c9e984dc62d7a190
Signed-off-by: Sonia Farrah <sonia.farrah@ericsson.com>
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/78765
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Hudson CI