docs: lead homepage with GitHub PR review story

[dcramer]

Summary

• move the GitHub PR review example directly below the hero
• keep skills central, but retell the page flow to show GitHub outcome first
• remove the 3-card value-prop boxes and tighten headline copy
• add a clarifying sentence after the PR mock and spacing below the mock

Verification

• pnpm --filter dex-docs build

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}