docs: Add USB install method to user guide

Update the ONIE user guide to include a section on how to install a
NOS using a USB thumb drive.
docs: Document ONIE CLI commands

Add a section to the documentation describing the handful of ONIE CLI
commands.

- onie-discovery-start
- onie-discovery-stop
- onie-nos-install
- onie-self-update
- onie-support
- onie-sysinfo

Mark these older commands as deprecated:

- install_url
- update_url
- support
docs: clarify machine name consistency

Made it clear that the ONIE machine name remains the same irrespective
of what ONIE mode you are in.
docs: details for developers contributing patches

Added a few more guidelines for developers when submitting patches to
the project.