[Docs] Add "python": executable file not found in $PATH in the Linux/Ubuntu troubleshooting/FAQ

[pedrominatel]

Add the "python": executable file not found in $PATH to the troubleshooting/FAQ guide.

See reference in: https://stackoverflow.com/questions/60762378/exec-python-executable-file-not-found-in-path

Solution tested: sudo apt install python-is-python3