Graphviz static rendering and docs

[lmeyerov]

Summary

• add plot_static engines for graphviz svg/png, graphviz-dot, and mermaid-code with position reuse support
• enable sphinx graphviz directive, refresh docs/notebook examples, and build docs image with graphviz/pygraphviz
• expand graphviz tests for new engines and doc snippets (rst/myst)

Testing

• PYTHONPATH=. uv run --python /usr/bin/python3 --with numpy --with palettable --with pandas --with pyarrow --with requests --with squarify --with typing-extensions --with packaging --with setuptools --with pygraphviz --with pytest python -m pytest graphistry/tests/plugins/test_graphviz.py -q
• DOCS_FORMAT=html VALIDATE_NOTEBOOK_EXECUTION=0 ./docs/ci.sh
• ./docs/validate-docs.sh docs/source/10min.rst docs/source/visualization/10min.rst docs/source/api/plugins/compute/graphviz.rst docs/source/gfql/spec/index.md