doc: change gerrit references to github

We're moving the project code to GitHub folks, so change references
in the documentation from gerrit over to GitHub:
https://github.com/zephyrproject-rtos/zephyr



Change-Id: Ic491a62ed43fc799eb5698e92435cb6eb4d89394
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>