Having recently moved from Zendesk to Jekyll, I'm still working out some of the kinks in the review process for non-technical folks. I've found that after a quick walkthrough of GitHub, most people are pretty comfortable leaving comments/making small changes in the web UI.
To show people what the content will look like once it's live on the site, I use a Netlify webhook to generate a deploy preview of the site, which is accessible at the bottom of every pull request. I typically add a comment on the PR that includes a direct link to the page on the deploy preview site that needs review with instructions on what I'm looking for.
Whenever commits are added to a PR, Netlify will trigger an update and re-deploy the preview. It's super handy for going back and forth and thus far has worked well - reviewers can view the Markdown content in-line on GitHub alongside the rendered version and add comments as they go.
It's not as efficient as I'd like it to be, but it does okay for now.