Test/document behaviour of non-consecutive header levels

[chrisjsewell]

As noted in executablebooks/MyST-NB#17:

One thing that should be checked/documented at the myst_parser level, is that the docutils.RstParser specifically prohibits transitions from 'non-consecutive' header levels, e.g.

# Title h1

### Title h3

If think with myst, this will currently end up being the same as:

# Title h1

## Title h2

I should check this, and decide how to handle it, e.g. would sphinx be happy with 'virtual' sections being added that don't have a <title> child.