Add specs of snippets from docs

spec/docs/mocks/mocks_spec.cr

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+class Phonebook
+  def find(name)
+    # Some expensive lookup call.
+    "+18005554321"
+  end
+end
+
+class Resolver
+  def initialize(@phonebook : Phonebook)
+  end
+
+  def find(name)
+    @phonebook.find(name)
+  end
+end
+
+Spectator.describe Resolver do
+  mock Phonebook do
+    stub find(name)
+  end
+
+  describe "#find" do
+    it "can find number" do
+      pb = Phonebook.new
+      allow(pb).to receive(find).and_return("+18005551234")
+      resolver = Resolver.new(pb)
+      expect(resolver.find("Bob")).to eq("+18005551234")
+    end
+  end
+end