feat(site): update landing, features, quickstart, and action pages for 0.5.0

[lostmartian]

What

• Updates marketing pages for the 0.5.0 release:
  • Header: adds 'Install App' link to https://github.com/apps/agentdiff-ci.
  • Quickstart: leads with agentdiff init and record --runs 3 statistical envelope capture.
  • Action: spotlights /agentdiff approve bot, hosted identity via token.agentdiff.app, and live 0.5.0 demo PR links.
  • Features: introduces statistical baselines, variance bands, and honest gate rules.
  • Compare & FAQ: highlights statistical envelopes and approve bot workflow.

Why

Keeps website messaging aligned with the shipped 0.5.0 release.

How

• Updated website/src/components/ and website/src/app/(site)/.
• Verified static build: pnpm build passing with 67 SSG routes.

Testing

pnpm build