Add support for captions of blocks replaced by ## title as="something"

[lubojr]

Currently the only way how to describe (add captions) on what the user is looking at in any section denoted by ## (eox-chart, eox-map or other) is to make another ## section, with as=div nav=false so that it does not appear in the navigation

We could maybe add a new caption element to simplify the possibility to add the caption for the previous section (replaced by as= logic)

[santilland]

maybe the section title (##) could be used as caption? it seems strange to have some text that is not used for anything apart from appearing in the navbar, which seems mostly is disabled manually as it is not wanted