Added the possibility of social provider extension via theme

Also started docs page

dev/docs/logic-theme-service.md

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+# Logic Theme Service
+
+
+#### Custom Socialite Service Example
+
+The below shows an example of adding a custom reddit socialite service to BookStack. 
+BookStack exposes a helper function for this via `Theme::addSocialDriver` which sets the required config and event listeners in the platform.
+
+The require statements reference composer installed dependencies within the theme folder. They are required manually since they are not auto-loaded like other app files due to being outside the main BookStack dependency list. 
+
+```php
+require "vendor/socialiteproviders/reddit/Provider.php";
+require "vendor/socialiteproviders/reddit/RedditExtendSocialite.php";
+
+Theme::listen(ThemeEvents::APP_BOOT, function($app) {
+    Theme::addSocialDriver('reddit', [
+        'client_id' => 'abc123',
+        'client_secret' => 'def456789',
+        'name' => 'Reddit',
+    ], '\SocialiteProviders\Reddit\RedditExtendSocialite@handle');
+});
+```