[core] Add "Get Started" Guide (#16)

This commit adds a "Get Started" guide for the mobile and desktop
version of FeedDeck. The guide describes the basic concepts and settings
of the app to make it easier to get started with the FeedDeck.

This is also used for the review process in the different app stores to
make it easier to get the app approved.

app/lib/widgets/settings/settings_info.dart

@@ -1,3 +1,6 @@
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 
 import 'package:package_info_plus/package_info_plus.dart';
@@ -139,6 +142,22 @@ class _SettingsInfoState extends State<SettingsInfo> {
             } catch (_) {}
           },
         ),
+        _buildItem(
+          'Get Started',
+          const Icon(Icons.help),
+          () {
+            try {
+              if (kIsWeb ||
+                  Platform.isLinux ||
+                  Platform.isMacOS ||
+                  Platform.isWindows) {
+                openUrl('https://feeddeck.app/get-started/desktop');
+              } else {
+                openUrl('https://feeddeck.app/get-started/mobile');
+              }
+            } catch (_) {}
+          },
+        ),
         _buildItem(
           'GitHub',
           const Icon(FDIcons.github),