Examples were supported with/without number suffix,
this caused problems using mypy for type checking because it attempted
to resolve imports such as `mathutils` to the example file.
Having only some examples numbered already complicated documentation
for conventions with examples - simplify extraction and number all.
After new AUD API changes from 2.8x what "buffer" function used to do
has now become "cache" function (it caches a sound into RAM). Therefore,
the basic aud example should call this new "cache" function instead of
"buffer" function.
Thanks to Michael-Z-Freeman for pointing out.
