doc: do not auto-generate doc/avoptions_(codecs|formats).texi

The contents of the two files have been hardcoded to codecs.texi and
formats.texi, so there is no use for generating the two files during build
time.

I kept the rules for generating them for future convenience.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/codecs: Cosmetics

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/codecs: Various updates

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