examples/muxing: set sample formats from list of codec supported sample formats

Avoid the need of tweaking, also show how to get list of supported sample
formats.
examples/muxing: honour distinction between encoder PTS timebase and stream timebase

Fix PTS set on the frame when encoding, which must be specified in the
encoder timebase or this will confuse the encoder.

When muxing the packet, the PTS/DTS generated by the encoder is then
rescaled to the stream timebase.
examples/muxing: reuse global audio frame

Simplify logic, avoid multiple unnecessary alloc/free operations.
examples/muxing: use av_frame_free() in place of av_free()
examples/muxing: set timestamps in output audio packet

In particular, fix trac ticket #3231.
doc: remove encoder/muxer/sinks docs from ffplay- and ffprobe-all

ffplay and ffprobe are read-only.

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