Add `intro` section placeholder to page template

[lennym]

This allows pages to define a more complex intro section than a single paragraph of text if required by defining a section placeholder that can be overridden in page templates if required.

Example

{{<step}}
  {{$intro}}
    {{#markdown}}my-markdown-content{{/markdown}}
  {{/intro}}
{{/step}}

[sulthan-ahmed]

👍