packet analyzer / doc: toml to manage dependency

The commit does the following:
- Manage Packet Analyzer python dependency by adding 'toml' file.
- Update documention to use python virtual environment for running the
  Packet Analyzer.

Issue-Id: SCM-6812
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Vineeth Raveendran <vineeth.raveendran@arm.com>
Change-Id: I9112ae2597fa90663ee77a65bd9c87d37bd61429