fix: convert code cells to markdown to remove epmcminer runtime dependency

GitHub's renderer may attempt to execute or validate code cells, failing
when epmcminer is not installed. Converts the three API example cells to
fenced code blocks in markdown with pre-rendered output shown below.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: julian-praesent