docs: sync README triton floor with setup.py and fix contributing install command

[kobihikri]

Summary

Two small documentation accuracy fixes:

1. README CUDA requirements said triton >= 2.3.0, but setup.py resolves triton>=2.3.1 — issue I have triton 3.3.0 but it shows this error : liger-kernel 0.5.8 depends on triton>=2.3.1 #682 shows a user hitting exactly this confusion. Updated the README to >= 2.3.1 so both sources agree.
2. docs/contributing.md step 2 had pip install . -e[dev], which isn't valid pip syntax (the guide even carries a workaround note for the error it causes). Replaced with pip install -e ".[dev]", matching AGENTS.md and the README.

Testing Done

Doc-only change. Verified setup.py line 17 (triton>=2.3.1) and that pip install -e ".[dev]" is the form AGENTS.md uses.

• Hardware Type: n/a (docs)
• run make test to ensure correctness — n/a, docs only
• run make checkstyle to ensure code style — n/a, docs only
• run make test-convergence to ensure convergence — n/a, docs only

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For transparency (per AGENTS.md's AI-assistant support): I used AI assistance to spot and draft this; I verified both lines against setup.py and issue #682 myself.