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-repl.md

@@ -0,0 +1,25 @@
+# nix repl
+
+> Start an interactive environment for evaluating Nix expressions.
+> See <https://nixos.org/manual/nix/stable/language/index.html> for a description of the Nix expression language.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-repl.html>.
+
+- Start an interactive environment for evaluating Nix expressions:
+
+`nix repl`
+
+- Load all packages from a flake (e.g. `nixpkgs`) into scope:
+
+`:lf {{nixpkgs}}`
+
+- Build a package from an expression:
+
+`:b {{expression}}`
+
+- Start a shell with package from the expression available:
+
+`:u {{expression}}`
+
+- Start a shell with dependencies of the package from the expression available:
+
+`:s {{expression}}`