`yosys-witness` prereq `click`. Yosys environment vars & `yosys --help` output. Removing Ubuntu/macOS version numbers/names. Hide `troubleshooting` page.
Replace leftover `opt` example source/images with examples specific to the `opt_*` pass. Currently has images for `opt_expr`, `opt_merge`, `opt_muxtree`, and `opt_share`. Also includes some other TODO updates.
Also some other tidy up.