fix(headings): keep math and footnote references in headings

[RovshanMuradov]

Fixes #436

Heading cleanup flattened everything to text, concatenating MathML presentation and TeX (x2x^2), and treated footnote references as permalinks and removed them. Headings stay plain text, but <math> and footnote references (sup[id^="fnref:"]) are now kept.

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## THE dependence on x2x^2 and x2x^2: method
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## THE dependence on $x^2$ and $x^2$: method