Search for <img> tag in notebook and optionally replace with markdown equivalent

[psychemedia]

Add a simple tool to look for explicit <img /> HTML tags in notebook markdown cells and optionally replace them with markdown image tags or MyST markdown, eg of the form:

```{image} Part%2001%20Notebooks/images/00_01_accessibility_display.png
:height: 150px
:name: image-example
```

This is hackfix relating to working around a longstanding open issue within the jupyterlab-myst extension: jupyter-book/jupyterlab-myst#64