Mermaid Usage Instructions Missing

## Description



This is a documentation-only issue, and I've spent some time digging to get to the bottom of this but there is nothing I've been able to find on the web, phind.com, or, importantly, Jupyter Lab's documentation.



1. [Here](https://jupyterlab.readthedocs.io/en/stable/user/extensions.html#id9) is the Installation Instructions for Extensions, they seem incomplete because they don't have an example of the syntax of how to install a plugin. While it may be as simple as pip installing, it's unclear how to treat the @jupyterlab/extension syntax.



2. Additionally, this [PR](https://github.com/jupyterlab/jupyterlab/issues/14788) has been merged and from what I can tell is out however even on the [latest](https://jupyterlab.readthedocs.io/en/latest/search.html?q=mermaid) documentation (as opposed to stable) and mermaid is listed as a core plugin, but without instructions.



It would be great to know some tips and tricks and usage instructions. For example, is this applicable in markdown files and notebook files? Is there a way to make the mermaid diagram source available while viewing a markdown cell. Often with plotting libraries you can see the code that generates the rendered plot. Is there something like that for a markdown cell. Is this also available on jupyter notebook, or just in jupyter lab?



Thank you.

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