Initial work on rom_sd

This module needs way reworked to just be a state machine. I was trying
to get way too tricky with it so I went back to the drawing board and
made a state machine diagram for it. The diagram is included with this
commit. I also moved the current collection of documentation to a doc/
folder, and added a second-long audio rom to test everything out once
the rom_sd is working.

lib/audio_buffer_interface.sv

@@ -0,0 +1,18 @@
+interface audio_buffer_interface;
+    logic [10:0] addra;
+    logic [7:0] dina;
+    logic clka;
+    logic ena;
+    logic address_half;
+
+    modport driver (
+        output addra, dina, clka, ena,
+        input address_half
+    );
+
+    modport receiver (
+        input addra, dina, clka, ena,
+        output address_half
+    );
+
+endinterface