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/sumprod.v

@@ -0,0 +1,12 @@
+module sumprod(a, b, c, sum, prod);
+
+  input [7:0] a, b, c;
+  output [7:0] sum, prod;
+
+  {* sumstuff *}
+  assign sum = a + b + c;
+  {* *}
+
+  assign prod = a * b * c;
+
+endmodule