[toastal]

GitLab also support AsciiDoc.

SourceHut unfortunately does not. You can go through the hoops of POSTing to its GraphQL API from CI a new README from any format to HTML.

[dnsmichi]

You can also embed diagrams and flowcharts with Mermaid into Asciidoc, not only in Markdown.

https://docs.gitlab.com/ee/user/asciidoc.html#mermaid

Examples for Mermaid layouts in the GitLab handbook: https://about.gitlab.com/handbook/tools-and-tips/mermaid/