Updates to documentation (#280)

Documentation updates (including moving of header files

lib_xua/doc/rst/using_adv_mixer.rst

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+
+Mixer
+~~~~~
+
+Since the mixer has no I/O the instantiation is straight forward. Communication wises, the mixer cores are inserted 
+between the `AudioHub` and Buffering core(s)
+
+It takes three channel ends as parameters, one for audio to/from the buffering core(s), one for audio to/from the 
+`AudioHub` core and another one for control requests from the `Endpoint0` core.
+
+The mixer task will automatically handle the change in mix count based on the current sample frequency (communicated
+via the data channels from the buffering task).
+
+An example of how the mixer task might be called is shown below (some parameter lists are abbreviated) ::
+
+    chan c_aud_0, c_aud_1, c_mix_ctl;
+    
+    par
+    {
+        XUA_Buffer(..., c_aud_0, ...);
+
+        mixer(c_aud0, c_aud_1, c_mix_ctl);
+
+        XUA_AudioHub(c_aud_1, ...);
+
+        XUA_Endpoint0(..., c_mix_ctl, ...);
+    }