Test for existence of H6 and H7 subheadings

What does this MR do?

This test is truly janky and MVC. It attempts to flag every H6 and H7 subheading, per the discussion in Note maximum Markdown heading depth for docs (!93783 - merged) that subheadings deeper than H5 are unstyled. (Also, just not good ideas in general.)

Adds two warning level rules to our rule set.

Notes:

• I tried using scope: heading but I couldn't get it to capture ###### or #######.
• scope: only supports one item at a time, so you can't concatenate into a single rule like scope: heading.h6, heading.h7
• We might want to file an issue with Vale - we can't be the only ones to have this idea
• The link needs to be updated after Note maximum Markdown heading depth for docs (!93783 - merged) merges

Related issues

Related to !93783 (merged)

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