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-flake.md

@@ -0,0 +1,24 @@
+# nix flake
+
+> Manage Nix flakes.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html>.
+
+- See documentation about what Nix flakes are:
+
+`nix flake --help`
+
+- Create a new flake (just the `flake.nix` file) from the default template, in the current directory:
+
+`nix flake init`
+
+- Update all inputs (dependencies) of the flake in the current directory:
+
+`nix flake update`
+
+- Update a specific input (dependency) of the flake in the current directory:
+
+`nix flake lock --update-input {{input}}`
+
+- Show all the outputs of a flake on github:
+
+`nix flake show {{github:owner/repo}}`