Updates code examples, removing `counter_outputs.ys` in favour of a single script. Also adds a .gitignore for the output file `synth.v`.
`example_synth.rst` still pending updated example.
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.