add mermaid support

i like the rendered diagrams from [mermaid](https://mermaid.js.org)

would be nice to add mermaid support for this viewer :-)

it seems that the newest/maintained markdown-it plugin [i found](https://www.npmjs.com/search?q=markdown-it%20mermaid&ranking=maintenance) are
(not [listed on the integrations page..](https://mermaid.js.org/ecosystem/integrations.html#editor-plugins))
- [markdown-it-textual-uml](https://github.com/manastalukdar/markdown-it-textual-uml) [npm](https://www.npmjs.com/package/markdown-it-textual-uml) (Aug 31, 2022 | 216 WeeklyDownloads)
- [@wekanteam/markdown-it-mermaid](https://github.com/wekan/markdown-it-mermaid) - [npm](https://www.npmjs.com/package/@wekanteam/markdown-it-mermaid) (Feb 21, 2023 | 199 WeeklyDownloads)
- [@agoose77/markdown-it-mermaid](https://github.com/agoose77/markdown-it-mermaid) [npm](https://www.npmjs.com/package/@agoose77/markdown-it-mermaid) (Sep 22, 2022 | 15 WeeklyDownloads)

the other option without plugin seems to be some [*hacky* script embedding](https://github.com/markdown-it/markdown-it/issues/781#issuecomment-1059009874)...

Start by reviewing the viewer's Markdown rendering integration and the linked markdown-it Mermaid plugin options. Done means Mermaid diagram blocks render correctly in the browser viewer without breaking existing Markdown rendering; the issue does not name specific files or tests to run.

Written by the indexing model from the issue text.