Adds "intro" key to front matter for intro paragraph

[brentryanjohnson]

• Several design mockups have a custom-styled opening paragraph.
• This PR adds a new intro key to relevant front matter (for pages that have already been merged) for the developers to hook onto in layout for the purpose of styling this intro paragraph.

Warning
This change is incomplete, since it requires the addition of the intro to page layouts. (see #170) #172 addresses this

Note
This change temporarily removes the opening paragraph of the relevant pages (since, if retained, the content would be duplicated when the engineers add the intro at the layout level).

I believe the design intention is to use the usa-intro class for this paragraph, but it will require custom font and color that is not default in USWDS.

I will also add the "intro" field to the relevant content docs in google drive

[mark-meyer]

Hi Ryan, this is super-helpful. I tagged you in a PR into this branch to add the intro text into the template.