Feat: Have supporting text on the canvas in order to guide users

[QiJin-Bouvet]

Some proposals for this feature:

• Show the supporting text in grey on both the card itself and the description area in the sidebar of the card
• Once user type something in the description area, the supporting text disappear, and user input shows in black
• Specific supporting text for each green card
• Specific supporting text for each type of cards: Main Activity card, Choice card, Sub Activity card, Waiting card