Summary
This MR introduces the following changes
- adding support for specifying a URL in the markdown placeholders
Example
It works just like the placeholders we already use, but for a url the syntax changes from §§§<filename>:<start>:<end>§§§ to §§§url="<url>":<start>:<end>§§§
The left side shows a placeholder with a URL specified. On the right side you see the inserted content of the file living at this URL after rendering the md file.
