manpage: add section about using mpv from programs and scripts

Give an overview over the various methods. I feel like I've written text
like this over and over again (compatibility.rst and
interface-changes.rst for example duplicate the list of mpv API
abstractions), but such is life in hell.

Use this in particular to strongly suggest not to parse terminal output.
This suggestion got lost or de-emphasized at some point (maybe when
removing MPlayer and "slave mode" references). Some of this text is
still there, but it can be considered "fine print" at best, that nobody
will see. Now we have it in a more prominent place. This is especially
important since MPlayer-style use of mpv still seems to be prevalent,
see for example #7153.

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