workflows: add check for docbook/md manual equality

we want to make sure that rendering the manual from markdown without
going through docbook produces (semantically) the same output as with
going through docbook. to ensure this we'll build the manual twice, run
each manual through html-tidy to generate a normalized form and diff
the normalized forms. we don't want to compare raw output because that
exposes us to a lot of whitespace we'd have to reproduce exactly in the
MD render.

this check may be relaxed even further in the future, but hopefully not
by much.

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