ddh-intro: add vertical space for list.

The addition of the `<!-- /summary -->` line seems to have confused the MarkDent processor which is quite flaky about the vertical whitespace.

I have tested this change in publisher and now get the correct rendering, but I don't know if:
- this breaks something else which depends on that hidden tag, or
- there are other places with wonky rendering because of the new summary tagging.

cc: @jdorweiler