Add contributing guidelines, DCO, and license files

Add CONTRIBUTING.md documenting how to submit contributions (via
tools@kernel.org), the dual licensing model (CC-BY-SA-4.0 for content,
GPL-2.0+ for code), DCO sign-off requirements, and the policy for
AI-assisted contributions (Assisted-by trailer required, Signed-off-by
and Co-developed-by prohibited for AI agents). Logos are noted as
copyright by their respective owners, used by permission.

Add the Developer Certificate of Origin v1.1 as DCO in the repository
root, and add the full license texts under licenses/.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Assisted-by: claude-opus-4-6-20250925