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-registry.md

@@ -0,0 +1,25 @@
+# nix registry
+
+> Manage a Nix flake registry.
+> See `tldr nix3 flake` for information about flakes.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-registry.html>.
+
+- Pin the `nixpkgs` revision to the current version of the upstream repository:
+
+`nix registry pin {{nixpkgs}}`
+
+- Pin an entry to the latest version of the branch, or a particular reivision of a github repository:
+
+`nix registry pin {{entry}} {{github:owner/repo/branch_or_revision}}`
+
+- Add a new entry that always points to the latest version of a github repository, updating automatically:
+
+`nix registry add {{entry}} {{github:owner/repo}}`
+
+- Remove a registry entry:
+
+`nix registry remove {{entry}}`
+
+- See documentation about what Nix flake registries are:
+
+`nix registry --help`