README quickstart, Colab badges, performance notes and community files

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• Docs only, no code changes
• README.md: Quickstart section before Requirements
  • installs via fetch-cuoptlink.py install -g "$(gamspy show base)" --cuda-runtime, then a minimal GAMSPy LP solved with solver="cuopt" (ran it locally, objective 1.4)
• Open in Colab badges for both notebooks in README.md and examples/README.md
  • also fixes the CUDA 13 notebook link that showed the CUDA 12 filename
• README.md: Performance section citing NVIDIA's Mittelmann LP benchmark (Oct 2024, H100), including its caveats (8/49 timeouts, small models)
• Community files: CONTRIBUTING.md (local build, the three test suites, PR rules), SECURITY.md (report to [email protected]), bug report issue form and config.yml linking the GAMS forum
  • checked that the YAML parses, the bug label exists and README anchors resolve
• Depends on Fix example notebooks for current release zips #21: until it's merged, the Quickstart command fails on GAMSPy (gamslib self-test) and the Colab notebooks fail on the solver config
• Not tested: notebooks in Colab, how the issue form renders on GitHub