How can I try out the new layout? How can I trigger the building process and see the result before merging?

The easiest might be to expose your fork of the doc, like @Inverle did in #8247

I think what I was doing is triggering the workflow manually via https://github.com/math-GH/FreshRSS/actions/workflows/jekyll-gh-pages.yml (on docs branch) and then it would be deployed at https://math-gh.github.io/FreshRSS/

(although you need to enable GitHub Actions deploying in https://github.com/math-GH/FreshRSS/settings/pages first)

Alternatively, here are brief instructions to run Jekyll locally: https://github.com/FreshRSS/FreshRSS/blob/edge/docs/README.md

I think what I was doing is triggering the workflow manually via https://github.com/math-GH/FreshRSS/actions/workflows/jekyll-gh-pages.yml (on docs branch)

great! That was the trick I missed before :)

I will link my page in the description above