nixos/manual: disallow docbook option docs

it's been long in the making, and with 23.05 out we can finally disable
docbook option docs and default to markdown instead. this brings a
massive speed boost in manual and manpage builds, so much so that we may
consider enabling user module documentation by default.

we don't remove the docbook support code entirely yet because it's a lot
all over, and probably better removed in multiple separate changes.

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