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/nix-classic.md

@@ -0,0 +1,29 @@
+# Nix classic interface
+
+> A classic, stable interface to a powerful package manager that makes package management reliable, reproducible, and declarative.
+> Some Nix commands such as `nix-build`, `nix-shell`, `nix-env`, and `nix-store` have their own pages. See also: `tldr nix`.
+> More information: <https://nixos.org>.
+
+- Search for a package in nixpkgs via its name:
+
+`nix-env -qaP {{search_term_regexp}}`
+
+- Start a shell with the specified packages available:
+
+`nix-shell -p {{pkg1 pkg2 pkg3...}}`
+
+- Install some packages permanently:
+
+`nix-env -iA {{nixpkgs.pkg1 nixpkgs.pkg2...}}`
+
+- Show all dependencies of a store path (package), in a tree format:
+
+`nix-store --query --tree {{/nix/store/...}}`
+
+- Update the channels (repositories):
+
+`nix-channel --update`
+
+- Remove unused paths from Nix store:
+
+`nix-collect-garbage`