Add public setup docs page

[kovyrin]

Summary

• Add a public docs page for Agent Secret setup and day-to-day use.
• Cover install, 1Password SDK setup, Bitwarden Secrets Manager token/source setup, project profiles, safe validation, troubleshooting, and FAQ.
• Link the docs page from the site nav/footer and add it to the sitemap.
• Start the next pending changelog section for the site docs change.

Validation

• mise exec -- scripts/checks/test-site-sitemap.sh
• mise exec -- scripts/checks/test-public-docs.sh
• mise exec -- npx --no-install markdownlint CHANGELOG.md
• mise run lint
• agent-browser desktop/mobile viewport checks at 1440px and 390px: no horizontal overflow
• agent-browser screenshots reviewed for the docs page