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-develop.md

@@ -0,0 +1,16 @@
+# nix develop
+
+> Run a bash shell that provides the build environment of a derivation.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-develop.html>.
+
+- Start a shell with all dependencies of a package from nixpkgs available:
+
+`nix develop {{nixpkgs#pkg}}`
+
+- Start a development shell for the default package in a flake in the current directory:
+
+`nix develop`
+
+- In that shell, configure and build the sources:
+
+`configurePhase; buildPhase`