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-run.md

@@ -0,0 +1,13 @@
+# nix run
+
+> Run an application from a Nix flake.
+> See `tldr nix3 flake` for information about flakes.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-run.html>.
+
+- Run a command whose name matches the package name from nixpkgs (if you want a different command from that package, see `tldr nix3 shell`):
+
+`nix run nixpkgs#{{pkg}}`
+
+- Run the default application in the flake in the current directory:
+
+`nix run`