Improve error-writing documentation

[Reefact]

Summary

This PR separates stable error documentation from runtime message guidance so developers can write precise, safe, and diagnosable errors without navigating overlapping pages. It also turns the best-practices page and FAQ into concise entry points that direct readers to the focused guides.

Type of change

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

Changes

• Rewrite the English and French error-documentation guides around one complete example and a review checklist.
• Add dedicated English and French guides for ShortMessage, DetailedMessage, and DiagnosticMessage.
• Document audience separation, sensitive-data boundaries, ErrorContext, RFC 9457 mapping, and localization with concrete good/bad examples.
• Replace the duplicated best-practices explanations with a compact project and pull-request checklist.
• Shorten the FAQ and make it direct readers to the focused documentation instead of repeating it.
• Keep all English and French pages structurally aligned and update their navigation links.

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 branch diff and cross-page navigation 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 counterparts of every changed or added English page are included. doc/README.fr.md itself did not require a change because the existing Writing Errors Guide remains the entry point and now links to the new message guide.

Related issues

None.