[DOCUMENTATION] Emphasize differences from real Robot API

[charleshong3]

The simulator API is different from the real robot in the Robot API in a few ways. We should emphasize these with disclaimers in the Help tab.

• In the real Robot API, the keyboard can’t be used with the Gamepad API
  • The provided skeleton code (in student_code_file.py) should use the Keyboard rather than Gamepad
• Robot.pick_up() and Robot.drop() are not real Robot API functions
• We should define the "device IDs" as global variables so it's easier to drop in real device IDs when the students get to the real competition.