Clarify markdown levels for nbpages

[mquevill]

In the cell about markdown levels, there's a disconnect between standard markdown levels and nbpages levels. Perhaps additional clarification about these might be helpful. For instance: In nbpages "title" will be 1.1, "section" will be 1.1.1, "subsection" will be 1.1.1.1, etc. but a markdown user might expect "title" to be 1, "section" to be 1.1, etc.

nbpages: (if we are inside example 1)

1.1 title
    1.1.1 section
        1.1.1.1 subsection
            ...

markdown:

1 title
    1.1 section
        1.1.1 subsection
            ...

https://github.com/ndcbe/cbe67701-uncertainty-quantification/blob/e224cc6c4684c931c2214c376ee29b36388076aa/notebooks/01.01-Contributed-Example.ipynb#L57-L78

[adowling2]

Closed by #11.