Update Cyton external trigger documentation

[philippitts]

There are several issues with the Cyton external trigger documentation.

• [ ] Instances of "may all your troubles disappear" should be removed.
• [ ] The references to the old OpenBCIHub should be removed.
• [ ] The documentation states that there is no noticeable delay between pushing the PROG button and seeing the digital pin response in the GUI, but there is a delay (from the Bluetooth communication). It's not a problem since timestamps are written on the device for the external trigger, but the documentation should be updated to explain this.
• [ ] We should add a section that introduces the Digital Read widget which makes it very easy to set the board to Digital Read mode.
• [ ] Remove "Brilliant!" in the "Utilize the on device push button" section. While I hope users find it brilliant, but we shouldn't assume so in the documentation.
• [ ] Review and follow the instructions on the page then make changes as needed.

[retiutut]

From #91

The Cyton External Trigger doc needs to be cleaned up to reflect what actually works and has been learned. It should also be updated to point to the OpenBCI Video Experiment doc (mentioned below).

https://docs.openbci.com/Cyton/CytonExternal/

Optoisolation and using a photodiode pressed against the screen are two functional methods.

We also have an experiment that uses a photodiode (involving visual stimuli of puppies and kittens). Maybe this doc could be changed slightly (title + mentioning it uses external trigger a little more) to further increase clarity. The purpose of this doc is to show one reliable use-case for an external trigger using a photodiode. This doc also highlights how to time-lock sync visual stimuli.

https://docs.openbci.com/Examples/VideoExperiment/