docs: refresh overview pages for keepers, min_usdc_out, and immutability

[collinsezedike]

Summary

• how-it-works.md: withdraw()'s documented signature was missing min_usdc_out (added by fix(vault): add min_usdc_out slippage guard to withdraw() #600). Added the new slippage-guard and contract-immutability security properties, and a missing section on the keeper system (accrual + migration), which wasn't documented anywhere despite being how the product keeps its yield promise over time.
• introduction.md: corrected an inaccuracy implying protocol switching only happens via manual admin action. The migration keeper does this automatically now; set_adapter is the manual fallback, not the mechanism.
• why-meridian.md: connected the "why an aggregator" pitch to the fact that rate-chasing is now automated, not just a deposit-time decision.

index.md was checked and needed no changes, its claims still hold.

Test plan

• prettier --check passes on all three files
• Verified withdraw()'s current signature and the keeper schedule directly against packages/contracts/vault/src/lib.rs and .github/workflows/keepers.yml before writing

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Project	Deployment	Actions	Updated (UTC)
meridian	Ready	Preview	Aug 27, 2026 7:16pm