docs: add AI Coding Assistants documentation

docs: add AI Coding Assistants documentation

Add guidance for AI assistants and developers using AI tools for kernel
contributions, per the consensus reached at the 2025 Maintainers Summit.

Create Documentation/process/coding-assistants.rst with detailed guidance
on licensing, Signed-off-by requirements, and attribution format. The
README points AI tools to this documentation.

This will allow coding assistants to easily parse these instructions and
comply with guidelines set by the community.

Link: https://lwn.net/Articles/1049830/
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251223122110.2496946-1-sashal@kernel.org>