Add contextual headers+footers in PDF

[chrispcampbell]

Currently the PDF version of a docs project is pretty barebones. It would be nice if we could include the page number on each page along with other contextual info, such as the document name and current section. This is especially helpful when a "chapter" spans multiple pages. For example, we could have the page header show the doc name on the left and the section name right justified like this:

En-ROADS User Guide                                              Background on En-ROADS Dynamics

[page content]

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