Makefiles now have `clean` target.
Also fixed top level makefile calls to images directory.
More yosys scripts instead of inline yosys commands in makefiles (which also means they can be included in the accompanying document when talking about the image generated).
Fixed another couple image generators that were still outputting pdf directly.
Fixed some hanging image references which hadn't been updated.
Adjusted some text related to images, and included a couple more intermediate images on `memdemo`.
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.
Krystine Sherwin