Docs: scripting_intro/show_intro

Adds two new `show` commands to `fifo.ys` for demo purposes.
Mention referencing named selections with `@<name>`.
Also adds a note to `example_synth` to point to the show intro.

docs/source/code_examples/fifo/fifo.ys

@@ -15,6 +15,8 @@ select t:*
 select -list
 select -set new_cells %
 select -clear
+show -format dot -prefix addr_gen_show addr_gen
+show -format dot -prefix new_cells_show -notitle @new_cells
 show -color maroon3 @new_cells -color cornflowerblue p:* -notitle -format dot -prefix addr_gen_hier
 
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