Support `setext` style md headings in menu item parsing

[MrArnoldPalmer]

Parsing md headers from the readme + api reference into a packages navigation items currently does not support setext style headings. This causes these items to not be displayed in the left nav of a package page.

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[addihorowitz]

@MrArnoldPalmer do you have an example you can share so that it will be easy to reproduce and resolve?

[MrArnoldPalmer]

settext headings are detailed in the github flavored markdown specification. As far as we know there is no package currently on construct-hub that uses them, and this should be superceded by a migration to a JSON format from jsii-docgen.

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