Rendered examples in documentation

[kildom]

Add plugin to markdown that shows rendered examples:

Example of `<b>` tag:

<!-- @image:bold -->
` ` ` xml
Example of <b>bold</b>.
` ` `
<!-- @end -->

The result is:

![bold](@image:bold)

Inline preview:
` ` ` xml
Example of <b>bold</b>.
` ` `
<!-- @preview -->

The plugin will do:

1. Create DocTML from code (wrap with <document> if needed)
2. Convert DocTML -> docx
3. Convert docx -> pdf (with python converter)
4. Convert pdf -> svg (with https://mupdf.readthedocs.io/en/latest/mutool-convert.html#mutool-convert)
5. If one page:
   • convert pdf -> temporary png (with mutool)
   • detect bounding box (just top and bottom, left and right should be on entire page)
   • if small bounding box compared to entire page (e.g. h < w), then set svg viewBox to bounding box plus borders (crop)
6. Put image where needed:
   • if inline: combine with code block
   • if non-inline: put as normal image
   • if cropped: use entire width
   • if not cropped: show page miniatures, expand on click