[codex] open get started guide safely

[Sean-Kenneth-Doherty]

Summary

• make the external Get started CTA open in a new tab like the other off-site links
• add rel="noopener noreferrer" to the GitHub guide link

Refs #501
Refs #347

Validation

• GITHUB_TOKEN=$(gh auth token) pnpm build
• confirmed generated dist/index.html renders the LinkButton anchor with target="_blank" and rel="noopener noreferrer"
• git diff --check main...HEAD

[Sean-Kenneth-Doherty]

Validated this head in a detached scratch worktree (e4d7fd74783cbcb4c7bc7e3fdf434acd51a34a44).

Commands/checks run:

GITHUB_TOKEN=$(gh auth token) pnpm build
rg -n '<a class="LinkButton" href="https://github.com/firstcontributions/first-contributions/blob/main/README.md" target="_blank" rel="noopener noreferrer"' dist/index.html
git diff --check

Result: pnpm build completed successfully, the generated homepage renders the Get started LinkButton with target="_blank" and rel="noopener noreferrer", and git diff --check was clean.

[Sean-Kenneth-Doherty]

Fresh validation against the current PR head (e4d7fd7):

• GITHUB_TOKEN=$(gh auth token) pnpm build -> Astro build succeeded, 2 pages
• confirmed generated dist/index.html renders the Get started LinkButton anchor with target="_blank"
• confirmed the same anchor includes rel="noopener noreferrer"
• git diff --check origin/main...HEAD -> clean

Generated .astro/dist output was restored after validation.