Updates to documentation (#280)

Documentation updates (including moving of header files

lib_xua/doc/rst/opt.rst

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+.. _sec_options:
+
+Options
+-------
+
+This section describes key options of ``lib_xua``. These are typically controlled using build time defines. 
+Where something must be defined, it is recommended this is done in `xua_conf.h` but could also be done in the application Makefile.
+
+For full details of all options please see  ::ref:`sec_api`.
+
+.. toctree::
+   
+    Strings <opt_strings> 
+    Code Location <opt_location>
+    Channel Counts and Sample Rates <opt_channels>
+    USB Audio Class Support <opt_audio_class>
+    Synchronisation <opt_sync> 
+    I2S/TDM <opt_i2s>
+    S/PDIF Transmit <opt_spdif_tx>
+    S/PDIF Receive <opt_spdif_rx>
+    MIDI <opt_midi>
+    PDM Microphones <opt_pdm>
+    Mixer <opt_mixer>
+    Direct Stream Digital (DSD) <opt_dsd>
+    USB Audio Formats <opt_audio_formats>
+    Other Options <opt_other>
+
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