Support `file:` URLs (and absolute paths) for images

The [markdown reference site](https://www.markdownguide.org/basic-syntax/#images-1) lists the common markdown syntax for images as the same as a link, i.e.
```
[link text](http://link.target)
```
but with a preceding !, e.g.
```
```

This should be relatively easy to implement: when matching that pattern in the source markdown, inject an HTML [`` tag](https://www.w3schools.com/html/html_images.asp) to the rendered HTML.

When I get time I plan on working on this.

The issue names no files or tests. Start by locating the markdown-to-HTML handling for the shown image syntax, including file: URLs and absolute paths; done means those inputs render as HTML image elements.

Written by the indexing model from the issue text.