docs: moving code examples

Code now resides in `docs/source/code_examples`.
`CHAPTER_Prog` -> `stubnets`
`APPNOTE_011_Design_Investigation` -> `selections` and `show`
`resources/PRESENTATION_Intro` -> `intro`
`resources/PRESENTATION_ExSyn` -> `synth_flow`
`resources/PRESENTATION_ExAdv` -> `techmap`,  `macc`, and `selections`
`resources/PRESENTATION_ExOth` -> `scrambler` and `axis`

Note that generated images are not yet configured to build from the new code locations.

docs/source/code_examples/selections/select.v

@@ -0,0 +1,15 @@
+module test(clk, s, a, y);
+    input clk, s;
+    input [15:0] a;
+    output [15:0] y;
+    reg [15:0] b, c;
+
+    always @(posedge clk) begin
+        b <= a;
+        c <= b;
+    end
+
+    wire [15:0] state_a = (a ^ b) + c;
+    wire [15:0] state_b = (a ^ b) - c;
+    assign y = !s ? state_a : state_b;
+endmodule