refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/administration/store-corruption.section.md

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+# Nix Store Corruption {#sec-nix-store-corruption}
+
+After a system crash, it's possible for files in the Nix store to become
+corrupted. (For instance, the Ext4 file system has the tendency to
+replace un-synced files with zero bytes.) NixOS tries hard to prevent
+this from happening: it performs a `sync` before switching to a new
+configuration, and Nix's database is fully transactional. If corruption
+still occurs, you may be able to fix it automatically.
+
+If the corruption is in a path in the closure of the NixOS system
+configuration, you can fix it by doing
+
+```ShellSession
+# nixos-rebuild switch --repair
+```
+
+This will cause Nix to check every path in the closure, and if its
+cryptographic hash differs from the hash recorded in Nix's database, the
+path is rebuilt or redownloaded.
+
+You can also scan the entire Nix store for corrupt paths:
+
+```ShellSession
+# nix-store --verify --check-contents --repair
+```
+
+Any corrupt paths will be redownloaded if they're available in a binary
+cache; otherwise, they cannot be repaired.