Huh - that seems a very basic missing feature in the cloud version. We use bog-standard self-hosted JIRA and markdown editing is basic working functionality. People also add mermaid diagrams/charts to the issue. As well as custom diagram plugins, excel sheets and a whole gamut of documents.
To my great consternation, I have not found this to be true in the cloud version:
https://jira.atlassian.com/browse/JRACLOUD-72631
Special thanks to Matt Lachman for keeping up the good fight every (business) day.