message: add default block to docs
diff: </:api>
  </Freestyle::Usage>
  +<Freestyle::Usage @name="Minimal CtaBlock Usage">
  + <:description>
  + You can use the default block for convenience.
  + </:description>
  + <:example>
  + <Boxel::CtaBlock @state="default" as |a|>
  + <a.ActionButton>
  + Do this
  + </a.ActionButton>
  + </Boxel::CtaBlock>
  + </:example>
  +</Freestyle::Usage>
  +
  <Freestyle::Usage @name="CtaBlock Explanation">
  <:description>
  