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>