docs(url-replacements): document auth behaviour with url replacements

[risu729]

Note

Enhances url_replacements documentation with clearer, safer guidance and auth details.

• Replaces corporate-specific examples with neutral example.com domains and anchors regex patterns with ^ for safer matching
• Adds a prominent warning that authentication headers are preserved and forwarded to the replaced URL; advises using start-anchored patterns
• Introduces an Authentication section explaining .netrc usage with replacements, precedence over env-derived tokens, and example config
• Minor example updates (e.g., updated mirrors/CDN examples) and clarification of processing order remain unchanged

^{Written by Cursor Bugbot for commit db20691. This will update automatically on new commits. Configure here.}

[Copilot]

Pull request overview

This PR enhances the URL replacements documentation by adding important security and authentication guidance, while also standardizing examples to use example.com domains.

• Adds comprehensive security warnings about credential leaking when using URL replacements
• Documents authentication behavior with .netrc files
• Updates all examples to use example.com domains instead of real company domains
• Adds the ^ anchor to regex patterns for better security

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[jdx]

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