Investigate checked-in assets

[pierre]

In https://github.com/killbill/killbill-docs/pull/499, the following files were checked-in. Do we still need them? Can they be upgraded?

• https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css
• https://cdn.staticaly.com/gh/afeld/bootstrap-toc/v1.0.0/dist/bootstrap-toc.min.css
• https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js
• https://cdn.staticaly.com/gh/afeld/bootstrap-toc/v1.0.0/dist/bootstrap-toc.min.js
• https://code.jquery.com/jquery-3.6.1.min.js