README and Gallery cosmetic changes + Renaming some examples

[guillaumebrg]

Change list:

• Renaming: gesture_recognition >> action_recognition. This demo supports non-gesture classes, e.g. nodding, shaking head, so I guess this is a better name.
• Renaming & Unifying: hand gesture event detection / gesture_detection >> gesture_control
• Recorded new GIFs because the "Action Recognition" and "Gesture Control" ones were too similar (a lot of swipes in both cases). GIFs were also re-ordered (looked better with the fitness GIF in the middle)
• Added a mosaic GIF to the "Gallery" section of the README
• Added a preview of the Virtual Race App to the gallery

[guillaumebrg]

You seem to have uploaded the weights for the gesture_control demo, which were just added to the SDK download page. Was that accidental?

That's accidental... Will remove and talk with Ingo to rename the folder

[guillaumebrg]

Ingo already made the changes on the website. Let's merge and address the remaining points in a follow up PR