nixos-render-docs: add Freezable class

for most of our data classes we can use dataclasses.dataclass with
frozen=True or even plain named tuples. the TOC structure we'll need to
generate proper navigation links is most easily represented and used as
a cyclic structure though, and for that we can use neither. if we want
to make the TOC structures immutable (which seems like a good idea)
we'll need a hack of *some* kind, and this hack seems like the least intrusive.

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