docs: clarify unsupported custom .npmrc keys and recommend alternatives

[maitrawebtech]

Adds clarification about unsupported custom configuration keys in .npmrc starting in npm v11.2.0.

Explains:

• Why unknown keys produce warnings
• That only officially supported npm config keys are recognized
• How to use package.json#config for package-level configuration
• How to pass arguments using --
• Recommendation to use environment variables instead of custom .npmrc keys

Closes #8843