web/security: put 5865d599c38850b2a1fd33fe197eb8fcc6c970a4 under () as the encoder lacked alpha support back then
style: add "warning" style similar to .info but has a red border

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
web: Add generate-doc.sh to generate makeinfo-based ffmpeg docs

Signed-off-by: Timothy Gu <timothygu99@gmail.com>