Allow custom help providers (#1259)

Allow custom help providers

* Version option will show in help even with a default command

* Reserve `-v` and `--version` as special Spectre.Console command line arguments (nb. breaking change for Spectre.Console users who have a default command with a settings class that uses either of these switches).

* Help writer correctly determines if trailing commands exist and whether to display them as optional or mandatory in the usage statement.

* Ability to control the number of indirect commands to display in the help text when the command itself doesn't have any examples of its own. Defaults to 5 (for backward compatibility) but can be set to any integer or zero to disable completely.

* Significant increase in unit test coverage for the help writer.

* Minor grammatical improvements to website documentation.

examples/Cli/Help/Program.cs

@@ -0,0 +1,19 @@
+using Spectre.Console.Cli;
+
+namespace Help;
+
+public static class Program
+{
+    public static int Main(string[] args)
+    {
+        var app = new CommandApp<DefaultCommand>();
+
+        app.Configure(config =>
+        {
+            // Register the custom help provider
+            config.SetHelpProvider(new CustomHelpProvider(config.Settings));
+        });
+
+        return app.Run(args);
+    }
+}