nix-collect-garbage: add page (#2090)

pages/common/nix-collect-garbage.md

@@ -0,0 +1,16 @@
+# nix-collect-garbage
+
+> Delete unused and unreachable nix store paths.
+> Generations can be listed using `nix-env --list-generations`.
+
+- Delete all store paths unused by current generations of each profile:
+
+`sudo nix-collect-garbage --delete-old`
+
+- Simulate the deletion of old store paths:
+
+`sudo nix-collect-garbage --delete-old --dry-run`
+
+- Delete all store paths older than 30 days:
+
+`sudo nix-collect-garbage --delete-older-than {{30d}}`