README: restructure with role-based documentation and guidelines

README: restructure with role-based documentation and guidelines

Reorganize README to provide targeted documentation paths for different user
roles including developers, researchers, security experts, and maintainers.

Add quick start section and essential docs links.

Signed-off-by: Sasha Levin <sashal@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251121180009.2634393-1-sashal@kernel.org>