docs: add API cost warning when making Space public

[octo-patch]

Fixes #604

Problem

When students set their duplicated Space to public, every visitor triggers inference API calls authenticated with the Space owner's token. If the token is linked to a paid HF Inference tier or an external paid API (e.g., OpenAI), this can silently accumulate significant costs for the student.

There is currently no warning about this risk in the tutorial steps where the secret token is configured.

Solution

Added a visible blockquote warning immediately after the "Save" step in units/en/unit1/tutorial.mdx, advising students to:

• Keep the Space private while experimenting
• Or monitor their API usage regularly if they choose to make it public

Testing

Documentation-only change; no code logic affected.