Nix: update and add documentation for nix subcommands (#9959)

* Nix: update and add documentation for nix subcommands

Updates the documentation on the new unstable Nix command & subcommands,
and adds a separate page for the Classic Nix commands.

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: axtloss <axtlos@getcryst.al>
Co-authored-by: CleanMachine1 <78213164+CleanMachine1@users.noreply.github.com>

pages/common/nix3-store.md

@@ -0,0 +1,25 @@
+# nix store
+
+> Manipulate the Nix store.
+> See also: `tldr nix-store`.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-store.html>.
+
+- Collect garbage, i.e. remove unused paths to reduce space usage:
+
+`nix store gc`
+
+- Hard-link identical files together to reduce space usage:
+
+`nix store optimise`
+
+- Delete a specific store path (most be unused):
+
+`nix store delete {{/nix/store/...}}`
+
+- List a contents of the store path, on a remote store:
+
+`nix store --store {{https://cache.nixos.org}} ls {{/nix/store/...}}`
+
+- Show the differences in versions between two store paths, with their respective dependencies:
+
+`nix store diff-closures {{/nix/store/...}} {{/nix/store/...}}`