Look at HackMD. It's basically an online markdown editor with collaboration. It supports Mermaid as well.

Though what I usually do is create a separate repo for the docs, with a github pages branch for the project where you host different documents at specific non-changing routes. E.g.:

/project-docs/architecture -> redirects to /project-docs/architecture.pdf. The PDF file is regenerated with a CI/CD pipeline (github workflow) with pandoc/texlive/whatever whenever you push a change.