[resources] Add LearnCpp (#64)

Author: Tsche

content/resources/guides.yaml

@@ -3,4 +3,7 @@ type: card_group
 id: guides
 title: C++ guides & FAQs
 description: All sorts of resources to improve your code
-cards: []
+cards:
+  - title: LearnCpp
+    path: https://www.learncpp.com/
+    description: LearnCpp.com is a free website devoted to teaching you how to program in modern C++