add ruff.toml with default config plus numpy docstrings

[galenseilis]

• Adding a ruff.toml.
• The rule set can be changed, but for the moment I want with the default given in the Ruff documentation with one added difference.
• The added difference is setting Ruff to use NumPy's pydocstyle rather than Googles or PEP257.
  • Discussion here seems to support this choice of convention.

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After there are some agreed rules for Ruff, I will start making non-breaking formatting changes.

[galenseilis]

#217