Select a standard pin layout for the examples and update README files.

 - Choose a standard pin layout for all the examples based on the pin connection table in the readme files.
 - Different pin layout in the example files could lead to not working examples.
 - Changes in the pin layout table to describe how to connect the NRF24L01 pins to different boards
 - Remove duplicate readme information in the RPi readme file and point to the default RPi/RF24/readme.md file.