Add slides (git 2025 day 1)

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 slides for the 1st day of the git 2025 course (and fix a typo)