docs: clarify pipx onboarding edge cases for RHEL9 and Alpine musl

[FabioLeitao]

Summary

• add docs-only onboarding guidance for two confirmed pipx failure paths post-1.7.4
• document RHEL9-family fallback to python3=3.9 and the explicit python3.12 install/pipx --python flow
• document Alpine/musl toolchain pre-step for current scikit-learn source-build fallback, and cross-link quickstart/readme/troubleshooting/matrix

Evidence references

• refs [docs][P2] TROUBLESHOOTING.md: missing section for multi-distro install failures (Void Linux xbps, Alma, Fedora, Oracle Linux) #492
• refs docs/onboarding: usuário pip install não tem caminho do install ao 1º scan (4 gaps de dogfooding) #1053

Test plan

• run ./scripts/lint-only.sh
• verify only 7 docs files were committed
• confirm no product code changes

Made with Cursor