Getting Started with Brushless Changes

[ArisMorgens]

Made some changes on the Getting started with the Crazyflie 2.1 Brushless tutorial.

1. Changed the number of Longer (silver) screws from 4 to 5.
2. Added that CW and CCW propellers are marked as “55-35R” and “55-35” respectively.
3. Corrected that the user should use short headers as shown in the video and not long ones.
4. Removed the "propeller direction" section so that all the propeller info (attaching, propeller direction, detaching) can be in the same section. Also added info about the propeller tool (a photo or a video would also be nice to add).
5. Added CW and CCW direction on the propeller image.
6. Added the use case of the long screws.
7. Added a guide on how to remove guards/legs. (a video would also be nice to add)

[gemenerik]

Thanks for improving the docs!