The image link check only looked at markdown and <img> tags, so two kinds
of references were never verified:
- MaixPy3 docs are Jupyter notebooks (.ipynb). The GIF->WebP conversion
updated the markdown pages but not the notebooks, and the check did not
notice.
- <video>/<source>/<audio> src and poster attributes.
Both kinds of breakage stay invisible on the live site because the COS
bucket still serves files that were removed from the repo.
Notebooks that are not valid JSON are reported on stderr instead of being
skipped silently. Also drop the two baseline entries fixed by #1048.
