Adds logic to migrate_qs2_to_qs3.py to convert:
- #showimg to Markdown image syntax .
- #linkimg to Markdown linked image syntax [](src).
- #ytvideo to HTML iframe embed for YouTube videos.
Image path processing handles absolute URLs and prepends /images/
for relative paths, aligning with previous qsgen2 behavior but
omitting hardcoded styling attributes for Markdown output.
