[codex] Add beginner gallery sample pack

[Kkkakania]

Summary

• Add a 30-second beginner path near the top of the README.
• Add docs/beginner_20_plots.md with 20 existing Python/MATLAB template entry points.
• Add examples/sample-pack/ with a small synthetic CSV and a free sample-pack README.
• Link the beginner sample pack to the complete examples/csv-workflow/ path that is now on main.

Closes #1.

Notes

The new sample pack is documentation and synthetic data only. It does not add copied paper figures, private data, binary MATLAB files, Office files, archives, or paid/commercial templates.

Validation

• Rebased on current main after PR [codex] Add CSV workflow example #3 merged.
• Ran the sample-pack CSV snippet with temporary matplotlib on PYTHONPATH; it produced a non-empty PNG in /tmp/codex-spfl-pr2-out.
• Ran scripts/check_publication_ready.py: passed.
• Ran scripts/check_release_state.py: passed for v2.1, 1000 templates, 79 palettes.
• Ran scripts/check_matlab_syntax.py: scanned 1015 .m files, 0 issues.
• Ran python -m pytest tests/: 1008 passed, 11 existing warnings.
• Ran git diff --check HEAD~1 HEAD: no whitespace errors after removing one trailing EOF blank line.

Cleanup

• Removed local pytest/cache directories before committing.
• Temporary validation directories under /tmp were not committed.