consider expanding Markdown image paths for Pandoc

[kevinushey]

Currently, image links of the form

![alt](~/path/to/file.png)

won't be handled by Pandoc, as the ~ prefix is not expanded:

pandoc: Could not fetch ~/scratch/plot.png
~/scratch/plot.png: openBinaryFile: does not exist (No such file or directory)
Error: pandoc document conversion failed with error 67

We might consider solving this with the use of a custom Pandoc filter, as described in http://pandoc.org/scripting.html

(previous discussion here: #1052)

[yihui]

This would resolve #811, but since (1) it is typically a bad idea to use absolute paths (2) it is more of a Pandoc issue (3) Pandoc has no interest in "fixing" it (jgm/pandoc#3135) and (4) the fix is not trivial, my conclusion is that we will just educate users on a case by case basis when this problem arises.

[github-actions]

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