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-edit.md

@@ -0,0 +1,12 @@
+# nix edit
+
+> Open the Nix expression of a Nix package in $EDITOR.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-edit.html>.
+
+- Open the source of the Nix expression of a package from nixpkgs in your `$EDITOR`:
+
+`nix edit {{nixpkgs#pkg}}`
+
+- Dump the source of a package to stdout:
+
+`EDITOR=cat nix edit {{nixpkgs#pkg}}`