* 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>