Add source links to one DeFi lesson

[beihaili]

Context

DeFi lessons are more credible when mechanism and risk claims link to canonical docs or reputable explainers.

Expected files

Pick one file under either:

• en/DeFiDeepDive/
• zh/DeFiDeepDive/

Acceptance criteria

• Add 3-5 source links for protocol mechanics, risk explanations, or definitions.
• Prefer official docs, protocol specs, reputable security explainers, or established educational resources.
• Keep the lesson beginner-readable.
• Avoid promotional referral links in the lesson body.

Definition of done

The selected DeFi lesson has stronger source support without becoming harder for beginners to read.

Verification

• Check each added link opens.
• Run npx prettier --check <changed-file> if formatting changes.

Good first issue candidate: yes.