miniaudio is updating it's amalgamation of dr_wav, etc. so that it's
all namespaced with "ma" which will make the amalgamated versions of
dr_libs entirely independent. There's no longer any need to mention
the decoding backends.

Documentation regarding stb_vorbis is removed so as to discourage
new users from using it. Support will not be removed until a
replacement Vorbis decoder can be amalgamated, but new users should
instead be guided to the libvorbis custom decoder in the extras folder.