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/CustomHelpProvider.cs

@@ -0,0 +1,30 @@
+using System.Linq;
+using Spectre.Console;
+using Spectre.Console.Cli;
+using Spectre.Console.Cli.Help;
+using Spectre.Console.Rendering;
+
+namespace Help;
+
+/// <summary>
+/// Example showing how to extend the built-in Spectre.Console help provider
+/// by rendering a custom banner at the top of the help information
+/// </summary>
+internal class CustomHelpProvider : HelpProvider
+{
+    public CustomHelpProvider(ICommandAppSettings settings)
+        : base(settings)
+    {
+    }
+
+    public override IEnumerable<IRenderable> GetHeader(ICommandModel model, ICommandInfo? command)
+    {
+        return new[]
+        {
+            new Text("--------------------------------------"), Text.NewLine,
+            new Text("---      CUSTOM HELP PROVIDER      ---"), Text.NewLine,
+            new Text("--------------------------------------"), Text.NewLine,
+            Text.NewLine,
+        };
+    }
+}