Hmm, I'd personally expect that from this short page people first click "many other advantages" to a longer page – which could perhaps see some improvements including links to all three relevant manuals/"components" (nix, nixos, nixpkgs).

Perhaps it could be in the menu like it is under nix. My motivation is that is easier to search in google for nixos manual than it is to navigate it from home page of nixos.

Menu bar should have Manual somewhere like nix page has.

there is "Learn" link in the toolbar

Right, that one has been added in the meantime, apparently.