Clarify distinction between errors and exceptions in documentation

[Reefact]

Summary

This PR refines the documentation to establish a clearer conceptual distinction between errors (as structured knowledge artifacts) and exceptions (as runtime mechanisms). The changes emphasize that FirstClassErrors transforms how errors are understood and documented, not the exception mechanism itself.

Key Changes

• Terminology refinement: Changed "exceptions" to "errors" in key taglines and descriptions across README files (English and French) to better reflect the library's focus on error semantics rather than exception mechanics
• Conceptual clarity: Updated core concept documentation to explicitly state that exceptions remain the signaling mechanism, while the library elevates the error data they carry into structured knowledge
• Design principle updates: Refined language in design principles to clarify that Outcome<T> allows errors to be "carried as structured data" rather than "used as structured error information," better distinguishing between the error itself and how it's transported
• FAQ improvements: Enhanced FAQ responses to clarify that the library doesn't replace exceptions but transforms the error knowledge they convey—from mere technical signals into documented knowledge units

Notable Details

All changes are documentation-only and maintain consistency across both English and French versions. The updates reinforce the library's core philosophy: treating errors as first-class knowledge artifacts while preserving exceptions as the underlying mechanism for signaling and propagating failures.