fix: improve HTML layer detection, various MD fixes

[vagenas]

Primary bug being addressed:
During HTML parsing (which BTW is also triggered by Markdown parsing when HTML elements are present), in case there were headings (<h*> tags) but no <h1> tags, all content was being marked as furniture.

Now the heuristic is being adapted so that:

• 🆕 if any heading is present (regardless of level h1, h2 etc.), the first one of them marks the beginning of the body; before that it's furniture
• else, i.e. if no headings are present at all, all content is marked as body

Markdown fixes:

• properly propagate section header levels
• improve handling of list subroots without text

[mergify]

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[PeterStaar-IBM]

Nice!

[ceberam]

A couple of comments:

• Following up with the discussion thread, would it be better to set the heuristic parameter to False by default? In this way, we ensure that no content is lost for users who are not aware of this feature
• In the heuristic rule, I would switch to ContentLayer.BODY as soon as we see an h* tag. Currently this only happens once we see <h1> and the specifications recommend that you do not skip heading levels but just in case we see documents starting with, say, <h2> and skipping <h1>.