Implement more elegant handling of page breaks

[mikej888]

Image width/heights, defined in CSS, and page breaks, defined in Markdown via CSS markup, are used to ensure a heading does not appear at the very bottom of a page or that half a page is empty because an image is too high for the space remaining.

Implement more elegant handling of page breaks to remove the need for using CSS markup in MarkDown.

This may require separate CSS depending on whether the HTML is to be deployed online or is being created as an intermediate step when creating PDFs.