replace FFmpeg with Libav in doc/
unless it stands for ffmpeg the program or is historic
doc: modify style for texi2html 1.78+
The generated HTML files are similar to the ones generated withtexi2html 1.56k used on the website.
Tested with texi2html 1.78 and 5.0. 1.78 is the minimal recommendedversion.
The removed @sp from the titlepage section were ignored until...
Simplify texi files naming: ff*-doc.texi -> ff*.texi.
Originally committed as revision 26006 to svn://svn.ffmpeg.org/ffmpeg/trunk