Add update-nixpkgs / diff-closures documentation

Adding the commands from !259 (merged) to the docs else I forget how to use it.

requested review from @jcalderone

assigned to @florian

added 2 commits

• 51a87401 - Update nixpkg filenames in docs
• 514cc6ea - Move update-nixpkgs / diff-closures docs to proper place

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changed title from Add update-nixpkgs micro howto to Add update-nixpkgs / diff-closures documentation

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approved this merge request

mentioned in merge request !289 (merged)

merged

mentioned in commit da8793e0

mentioned in merge request !293 (merged)