avformat/hlsenc: added segment filename option

This option allows segment filenames to be specified. Unless -hls_flags
single_file is set the filename is used as a string format with the
segment number.

Example:
ffmpeg -f lavfi -i testsrc -c:v h264 -map 0 -hls_segment_filename
bar%03d.ts foo.m3u8

Signed-off-by: Christian Suloway <csuloway@globaleagleent.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc: Do not use the headings as links to TOC anchors

Instead, use FontAwesome icons (if configured to be this way) or separate
text.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc: html: Use native viewport

Fixes displaying docs on small-screen devices.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>