Fix line width for sections without <h2>

When starting a document with something other than a `<h2>`, the body of the section was being incorrectly displayed as full width. This commit wraps everything in the body up until the first `<h2>` in a div allowing us to style it. [Ticket](https://trello.com/c/aUmow68N/267-bug-line-wrapping-on-manuals).