It said that the CC would not automatically be charged at the end of the trial.

However, there's no way to cancel/close your account before the end of the trial and there's no way to remove billing information either.

I ended up emailing support@ghost.org

The reason I cancelled is because the only theme I found I wanted to use (Doks) was $149 per domain.

I found that I can have Cloudflare Pages auto deploy my private Github repo's markdown and render with Docsify (which doesn't need a build step). I still used a build step to automatically generate _sidebar.md and populate it with links based on tags found in each markdown file.