Add literate notebook examples gallery

[gparadis]

Summary

• Adds real literate Jupyter notebooks for the synthetic and generated 2020 FABLE examples under examples/notebooks/.
• Links the notebooks from the Sphinx Examples Gallery as downloadable .ipynb files.
• Documents the notebook example contract and updates notebook/source-tree import guidance to find the repo by pyproject.toml instead of a clone directory name.
• Adds tests for notebook JSON/kernel metadata, markdown/code structure, stored outputs, synthetic notebook execution, and the static FABLE validation boundary.
• Bumps the release target to modelwright==0.1.0a6.

Verification

• scripts/bootstrap_dev_env.sh
• .venv/bin/python -m ruff check .
• .venv/bin/python -m pytest -vv passed with 167 passed and 1 skipped.
• .venv/bin/python -m pytest -vv tests/test_examples.py passed with 6 passed after the repo-root guidance fix.
• .venv/bin/sphinx-build -b html docs _build/html -W
• .venv/bin/python scripts/verify_docs_theme.py _build/html
• scripts/check_release_artifacts.sh passed; clean wheel install imported modelwright 0.1.0a6 and CLI help smoke passed.

Closes #187.
Closes #185.
Closes #186.
Closes #184.
Closes #188.
Refs #183.
Refs #189.