Document which plugin is required for Mermaid rendering

By default mermaid diagrams are inserted as structured diagram macros. However there is no documentation specifying which confluence plugin will render that macro. Can the documentation please be updated to clarify which plugin should be installed.

Deferred rendering of Mermaid diagrams was a user-contributed feature. (We only use the mode when PNG/SVG images are generated from the Mermaid source, and uploaded to Confluence as image attachments. This is what we have covered in integration tests.) From what I can see in the source code, a structured-macro with a name of macro-diagram is produced, which Confluence (or a Confluence plugin) would be expected to render on the server-side. If you find out what configuration settings and/or plugins you need to make this approach work, I am more than happy to extend the documentation. (Some information might be available in the original pull request.)

I found a reference to the plugin in the original PR: https://github.com/hunyadi/md2conf/pull/41/files

It's this macro: https://atlasauthority.atlassian.net/wiki/spaces/MARKDOWNCLOUD/pages/2946826241/Diagram+Macro

Provided by this plugin: https://marketplace.atlassian.com/apps/1211438/markdown-html-plantuml-latex-diagrams-open-api-mermaid

Unfortunately, the plugin is > $2k per year for me to install.