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-search.md

@@ -0,0 +1,17 @@
+# nix search
+
+> Search for packages in a Nix flake.
+> See `tldr nix3 flake` for information about flakes.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-search.html>.
+
+- Search `nixpkgs` for a package based on its name or description:
+
+`nix search {{nixpkgs}} {{search_term...}}`
+
+- Show description of a package from nixpkgs:
+
+`nix search {{nixpkgs#pkg}}`
+
+- Show all packages available from a flake on github:
+
+`nix search {{github:owner/repo}}`