The main motive of this commit is to make the offline copy of the HTML doc as
pretty as the online (ffmpeg.org) one. This also introduced new styles for
<h2> to <h4>.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
The attribute container of the top-level div is changed from "class" to
"id" to match the website CSS.
Improve consistency between website docs and local documentation style.
