add flutter day1 link

What does this MR do

• Add new data (course edition, post, etc.)
  • I tested locally
  • I read the README.md
• Fix an issue
• Add a new data type
  • I added the schema in schemas/
  • I tested locally
• Merge develop into master
  • I checked that the preview is as intended
• Other

Description

Add Flutter day 1 slides to site

courses/2024-05-flutter.yml

@@ -8,7 +8,7 @@ schedule:
     room: 3.0.3
     topics:
       - title: "Introduzione a Flutter"
-        # slides: ""
+        slides: "https://slides.poul.org/2024/flutter/1-introduction"
   - date: 2024-05-22 17:15
     room: 9.0.1
     topics: