docs: render code output in evaluation walkthrough with markdown-exec

[knutdrand]

Summary

• Add markdown-exec plugin so Python code blocks in the evaluation walkthrough execute during mkdocs build and display their output inline
• Blocks with print() output render as text; the DataFrame block uses to_markdown() for a native table
• Update CLAUDE.md and writing_building_documentation.md with guidelines for exec="on" blocks

Test plan

• uv run mkdocs build succeeds, rendered HTML shows code output
• make test-docs passes (mktestdocs skips exec blocks)