move diff-hook examples to nix.conf

[fricklerhandwerk]

Problem

Verifying Build Reproducibility adds information on top of regular diff-hook documentation, which is hard to find on the web and inaccessible on the command line through man nix.conf.

Proposal

Move that page to the documentation on diff-hook and use a pure Nix example with e.g. builtins.currentTime, and delete the page.

Depends on #7981

Important
add a redirect on nixos.org, otherwise existing links will break

Priorities

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