[Repo Assist] docs(getting_started): update install.rst for Python 3.9+, poetry workflow, and py-why links

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant.

Problem

The installation documentation in docs/source/getting_started/install.rst was significantly outdated:

1. Wrong Python version: stated "Python 3.6+" but the project has required Python 3.9+ for years
2. Non-existent file: referenced pip install -r requirements.txt but no requirements.txt exists — the project uses Poetry
3. Outdated dev setup: pip install -e . is not the recommended development workflow; Poetry is
4. Deprecated pip flags: --global-option=build_ext --global-option="-I..." for pygraphviz is deprecated in modern pip
5. Stale Microsoft link: conda section linked to github.com/microsoft/dowhy/issues/197 — repo moved to py-why
6. Convoluted Azure ML dev section: 8-step manual process to create and chmod a folder was confusing and unnecessary

Fix

• Correct Python version requirement: 3.6+ → 3.9+
• Replace pip install -e . dev instructions with the standard Poetry workflow (poetry install -E "plotting")
• Remove the dead pip install -r requirements.txt hint
• Replace deprecated --global-option pip flags for pygraphviz with plain pip install pygraphviz
• Fix conda issues link: microsoft/dowhy → py-why/dowhy
• Simplify Azure ML dev section to %pip install git+https://github.com/py-why/dowhy.git

Test Status

Documentation-only change — no Python code changed.

• ✅ flake8 not applicable (RST file)
• ✅ black/isort not applicable (RST file)
• ✅ Content verified against pyproject.toml (Python ≥3.9, poetry-based install)

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