docs: convert manpage contents into reStructuredText markup

docs: convert manpage contents into reStructuredText markup

The manual page was written directly in the low-level troff markup
used by manpages. Convert the contents into reStructuredText markup
and start generating the installed manual page from that instead. The
new documentation added in this commit also contains multiple content
updates compared to the old manual page contents.

The rst2man tool from Docutils is now needed to generate the manpage.
A new Makefile target "install-no-man" can be used to install the
binary without the manpage.

Also remove the now pointless $(MPLAYER) configuration variable that
was still used in Makefile manpage rules, and remove the corresponding
configure switches --enable-mplayer and --disable-mplayer.

The documentation added in this commit has been worked on by multiple
people:

Clément Bœsch <ubitux@gmail.com>
initial conversion of the old manpage text into RST markup

Uoti Urpala <uau@mplayer2.org>
many edits, modifications and fixes

wm4
new documentation for some added options/features

cantabile
fixes to some erroneous/outdated entries