Finalize documentation comparison and navigation

[Reefact]

Summary

This PR completes the documentation-improvement series with a concrete, neutral comparison against other error-handling libraries and a bilingual documentation map organized by reader intent.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Analyzer / diagnostic change
• Tests
• Documentation
• Build / CI / tooling

Changes

• Rewrite the English and French library-comparison pages around one shared payment-decline scenario.
• Compare ErrorOr, FluentResults, and FirstClassErrors by primary abstraction, operational scope, documentation responsibilities, and decision criteria.
• Date the comparison and link to the official upstream repositories so time-sensitive claims can be rechecked.
• Replace absolute feature claims with neutral distinctions such as built in, application-defined, or outside the library's main scope.
• Add English and French documentation maps organized by reader intent: discovery, model understanding, error authoring, application usage, delivery and operations, pipeline extension, and reference lookup.
• Keep the map limited to stable primary entry pages so this PR remains independently mergeable and introduces no links to pages that exist only in other open documentation PRs.
• Keep the English and French structures and navigation links aligned.

Testing

• dotnet build FirstClassErrors.sln
• dotnet test FirstClassErrors.sln
• Analyzer tests pass (FirstClassErrors.Analyzers.UnitTests)

No build or test command was run because this PR changes Markdown documentation only. The comparison was reviewed against the current public ErrorOr and FluentResults documentation, and the branch diff and documentation links were reviewed manually.

Documentation

• Public API / error documentation updated
• README / doc/ updated
• French translation (doc/README.fr.md) updated if user-facing behavior changed
• No documentation change required

The French counterpart of every changed or added English page is included. The root README files are intentionally unchanged in this independent PR because the earlier documentation batches introduce additional focused pages on separate branches; the new map uses only primary entry pages and can be linked from the final merged README without creating temporary broken links.

Related issues

None.