docs: begin writeup of guard

While writing this I realized that this really should be part of part_3_workflows.rst, so it'll eventually make it into there. But it's recorded in its unpolished form for now.

samples/rake-guard/src/characters.rb

@@ -0,0 +1,8 @@
+require 'squib'
+
+Squib::Deck.new do
+  background color: :white
+  text str: "Built at #{Time.now}"
+  svg file: 'robot-golem.svg'
+  save_png prefix: 'character_'
+end

samples/rake-guard/src/skills.rb

@@ -0,0 +1,7 @@
+require 'squib'
+
+Squib::Deck.new do
+  background color: :white
+  text str: "Built at #{Time.now}"
+  save_png prefix: 'skill_'
+end