Add PRU Cookbook

assigned to @lorforlinux

@jkridner All the PRU CookBook content has been added to docs.beagleboard.io I have pushed final cleanup commit for this, closing this until further issues found.

https://git.beagleboard.org/docs/docs.beagleboard.io/-/commit/4b27daa52e8197b1b4991a14241af6a2f4edf55e

closed