Move "help" to its own section under Preview

[coolaj86]

Considerations:

• help should remain be visible "above the fold"
• help is actually distinct from Preview (which is becoming more apparent as we use this more)
• help can be quite large

I think what we ought to do is to move the help to something like this:

<details><summary>View Help</summary>
<span data-id="help-output"></span></details>

Then when it's visible it will be completely visible, still prominent, but also not obscuring the arguably more important Preview.

It's a relatively minor change.