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/opt_expr.ys

@@ -0,0 +1,17 @@
+read_verilog <<EOT
+
+module uut(
+    input a,
+    output y, z
+);  
+    assign y = a == a;
+    assign z = a != a;
+endmodule
+
+EOT
+
+copy uut after
+opt_expr after
+clean
+
+show -format dot -prefix opt_expr_full -notitle -color cornflowerblue uut