home-manager: add init command to main tool

The init command is essentially the old install script but integrated
into the home-manager tool. This simplifies things slightly since we
can use the existing code infrastructure.

The init command is Nix flake aware in the sense that, if we detect
that the user's Nix setup supports flakes, then we also create an
initial `flake.nix` file.

Finally, we update the installation instructions for the Nix flakes
standalone setup to use the new init command.

Zsh completion update provided by Anund <anundm@gmail.com>.

docs/release-notes/rl-2305.adoc

@@ -30,6 +30,14 @@ The old location will continue to work but using it will trigger a warning messa
 We changed the default configuration location to avoid confusion about
 which files belong to Home Manager and which belong to Nixpkgs.
 
+* The `home-manager` tool now offers an `init` command.
+This command can be used to generate an initial Home Manager configuration,
+and optionally also activate it.
+The recommended installation method for a standalone Home Manager setup
+with Nix flakes uses this new command.
+The standard installation method remains the same but uses the new command internally.
+See <<sec-flakes-standalone-unstable>> for more.
+
 [[sec-release-23.05-state-version-changes]]
 === State Version Changes
 
@@ -45,5 +53,5 @@ These changes are only active if the `home.stateVersion` option is set to "23.05
 - <<opt-wayland.windowManager.sway.config.floating.titlebar>>
 --
 +
-now default to `true` which is consistent with the default values
-for those options used by `i3` and `sway`.
+now default to `true` which is consistent with the default values for
+those options used by `i3` and `sway`.