doc/encoders: add doc for AAC encoder

Thanks-to: Kostya Shishkov <kostya.shishkov@gmail.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
(cherry picked from commit 0e11790cf7eef3b0f38a64486da1e3fb8c7f14b8)

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