[GreedyC]

Summary

Closes #718.

Replace the removed setup.py install / setup.py develop commands with python -m pip install . / python -m pip install -e . in the README and three installation/guide pages. Update CONTRIBUTING to install the declared test extra with python -m pip install -e ".[test]" and run python -m pytest.

Validation

• Normal and editable source installs succeeded in separate clean Python 3.12 environments using the documented pip commands. Import and a small Fire command were verified outside the checkout, including the expected normal versus editable import paths.
• The exact editable test-extra command succeeded; the full suite passed: 273 tests.
• Pylint still reports existing source diagnostics (9.91/10); this docs-only diff leaves every file under fire/ identical to upstream. It is not a clean lint result.
• No obsolete setup.py references remain in the five edited instruction files; git diff --check passes.
• Copilot pre-review found no concrete introduced defect.

No runtime or dependency changes. The existing account CLA check on #706 is successful; no new agreement was accepted for this contribution.