Fix emoji rendering: ":rocket:" and ":loudspeaker:" should display as actual emojis in Unit 1

[delmalih]

Location: Unit 1 main page URL: https://huggingface.co/learn/diffusion-course/unit1/1

Issue: Two emoji codes are displaying as plain text instead of rendering as actual emojis.

1. Rocket emoji in "Start this Unit" section:

• Current: Start this Unit :rocket:
• Expected: Start this Unit 🚀

2. Loudspeaker emoji in Discord notice:

• Current: :loudspeaker: Don't forget to join the Discord...
• Expected: 📢 Don't forget to join the Discord...

Suggested fix: Replace :rocket: with 🚀 and :loudspeaker: with 📢 in the markdown file.