doc: rename guide to 'Nixpkgs Manual'
doc: rename guide to 'Nixpkgs Manual'

For consistency with 'NixOS Manual' and 'Nix Manual', to better match what it's
often called in practice, and to match its URL and HTML title.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs