feat(guide): add How it works page

[RUKAYAT-CODER]

Summary

Adds the missing "How it works" guide as a dedicated page at /how-it-works, accessible from the sidebar navigation.

Changes

• src/app/(dashboard)/how-it-works/page.tsx: new page with 5 numbered process steps (Submit ? Verify ? Dispute ? Rewards ? Identity) and an FAQ section covering wallet, tokens, timelines, and Worldcoin privacy
• src/components/layout/Sidebar.tsx: added "How it works" nav link using HiOutlineQuestionMarkCircle icon, placed above Documentation in the resources section
• src/tests/components/how-it-works.test.tsx: invariant tests verifying heading, all 5 steps, FAQ section, and at least one FAQ answer are rendered

closes #188