Moved icon replacement code to partial

[lamATnginx]

Proposed changes

Motivated by the fact that the replacement code can be used outside of just _default/docs.html, particularly banners or nav headers.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[github-actions]

✅ Deploy Preview will be available once build job completes!

Name	Link
😎 Deploy Preview	https://frontdoor-test-docs.nginx.com/previews/nginx-hugo-theme/266/

---

[danielledeleo]

Nice :)