docs: improve grammar in CONTRIBUTING.rst

[TianHengZhuang]

Summary

Polishes two awkward sentences in docs/CONTRIBUTING.rst (target branch: develop):

1. Opening sentence: clearer punctuation and article usage.
2. Coding guideline intro: "We believe to keep" → "We believe in keeping"; "less place" → "fewer places".

Testing

Documentation-only change (reStructuredText).

[codspeed]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing TianHengZhuang:docs/fix-contributing-grammar (e280273) with develop (95795cc)}

[waketzheng]

• Docs and pre-commit config only; no source code changes.
• The .pre-commit-config.yaml change only adds manual-stage hooks
  (mypy, bandit, build, twine) to support the documented
  uvx prek run --all-files --group check command. It does not affect
  existing hooks or default behavior.