Rendering of pages not in pages

[mortenpi]

Currently, if we have an .md file that is not included in the pages keyword, we still run the expanders etc., on it (e.g. executing all the at-blocks), but we don't render it in the HTML output. Would be good to be consistent with this -- either run & render, or do neither.

If we would enable rendering of these pages, they would end up as hidden HTML files that are outside of the normal navigation (since the nav. sidebar is defined by pages). Not sure what to do with them in the LaTeX output though -- create an appendix?