Disallow links in section headers

This is quite straightforward to implement, since we already had a
function `docToHtmlNoAnchors` (which we used to generate the link in the
sidebar "Contents").

This breaks test `Bug387`, but that test case has aged badly: we now
automatically generate anchors for all headings, so manually adding an
anchor in a section makes no sense. Nested anchors are, as pointed out
in #1054, disallowed by the HTML standard.

Fixes #1054

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