This changes the default configuration location for Home Manager
configurations from
$XDG_CONFIG_HOME/nixpkgs
to
$XDG_CONFIG_HOME/home-manager
The old location is still supported but using it will trigger a
warning message.
Fixes#3640
In the manual:
* Add chapter Nix Flakes
* Add links to sections of chapter "Using Home Manager"
In README.md:
* Remove section "Nix Flakes"
* Add manual reference at the buttom of section "Installation"
