Support custom commands via logical theme system

Added initial work to support registering commands through the logical
theme system. Includes docs changes and example.

Not yet covered via testing.

app/Console/Kernel.php

@@ -2,8 +2,11 @@
 
 namespace BookStack\Console;
 
+use BookStack\Facades\Theme;
+use BookStack\Theming\ThemeService;
 use Illuminate\Console\Scheduling\Schedule;
 use Illuminate\Foundation\Console\Kernel as ConsoleKernel;
+use Symfony\Component\Console\Command\Command;
 
 class Kernel extends ConsoleKernel
 {
@@ -35,6 +38,13 @@ class Kernel extends ConsoleKernel
      */
     protected function commands()
     {
+        // Default framework command loading from 'Commands' directory
         $this->load(__DIR__ . '/Commands');
+
+        // Load any user commands that have been registered via the theme system.
+        $themeService = $this->app->make(ThemeService::class);
+        foreach ($themeService->getRegisteredCommands() as $command) {
+            $this->registerCommand($command);
+        }
     }
 }

app/Theming/ThemeService.php

@@ -3,11 +3,17 @@
 namespace BookStack\Theming;
 
 use BookStack\Auth\Access\SocialAuthService;
+use Symfony\Component\Console\Command\Command;
 
 class ThemeService
 {
     protected $listeners = [];
 
+    /**
+     * @var Command[]
+     */
+    protected $commands = [];
+
     /**
      * Listen to a given custom theme event,
      * setting up the action to be ran when the event occurs.
@@ -43,6 +49,22 @@ class ThemeService
         return null;
     }
 
+    /**
+     * Register a new custom artisan command to be available.
+     */
+    public function registerCommand(Command $command)
+    {
+        $this->commands[] = $command;
+    }
+
+    /**
+     * Get the custom commands that have been registered.
+     */
+    public function getRegisteredCommands(): array
+    {
+        return $this->commands;
+    }
+
     /**
      * Read any actions from the set theme path if the 'functions.php' file exists.
      */