add pb tag to editor & exports

[saracarl]

One option -- the one we like the best -- would be for us to add a (page break) tag that is similar to the (column break) tag in our editor tags, as explained here: https://content.fromthepage.com/project-owner-documentation/advanced-editor/The advantage to that approach is that it would be handled appropriately in all the export formats -- marked with tags in TEI, as a div in HTML, a text export would have the same number of empty lines as we use to separate pages in a work export. We don't actually support that yet, but it has been requested and we'd be willing to add it. (You could also tell people to type it in -- you don't need the buttons for the tag to be a reasonable approach, and we'd handle it correctly once we supported the button.

once we do this we should create a documentation page to answer the question "what to do with multiple pages on a single image"