Docs: working on opt 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.

docs/source/code_examples/opt/Makefile

@@ -0,0 +1,19 @@
+PROGRAM_PREFIX :=
+
+YOSYS ?= ../../../../$(PROGRAM_PREFIX)yosys
+
+DOT_NAMES = opt_share opt_muxtree opt_merge opt_expr
+
+DOTS := $(addsuffix .dot,$(DOT_NAMES))
+
+dots: $(DOTS)
+
+%_full.dot: %.ys
+	$(YOSYS) $<
+
+%.dot: %_full.dot
+	gvpack -u $*_full.dot -o $@
+
+.PHONY: clean
+clean:
+	rm -f *.dot