Add WS2812B-Mini (NeoPixel Mini 3535 RGB LED)

This adds a new footprint for the WS2812B-Mini, a 3.5 by 3.5 mm NeoPixel RGB LED.

See also:

• kicad-packages3D-generator!35 (merged) for the 3D model generator (with some measurements)
• kicad-packages3D!953 (merged) for the 3D model
• !2896 (merged) for the nano version of the neopixel

I created it my shrinking the silkscreen and courtyard by 0.75 mm ((5.0 - 3.5) / 2) and placing the pads acconding to an older datasheet.

Note:

• All other WS2812 LEDs have datasheets linked to Adafruit, but I cannot find these particular LEDs there, it seems that Adafruit is using SK2812 in this form factor instead.
• I find the number 1 (to denote the pin number) on the silkscreen a bit weird. I would prefer to remove it, but it's probably there to avoid confusion with marking usually being set for pin nr. 1 rather than 3. I left it there for consistency with WS2812B_PLLC4