Improve testing and operational documentation

[Reefact]

Summary

This PR separates everyday error assertions from deterministic occurrence testing, and separates catalog delivery from runtime logging integration. The resulting guides give each concern a focused mental model, concrete examples, and review checklists without changing library behavior.

Type of change

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

Changes

• Rewrite the English and French testing guides around ShouldSucceed(), ShouldFail(), ErrorAssertion, and targeted behavioral assertions.
• Add dedicated English and French deterministic-testing guides for Clock, InstanceIds, custom sources, snapshots, scope lifetime, and parallel-test behavior.
• Rewrite the English and French operational-integration guides as end-to-end catalog generation and publication workflows.
• Add complete GitHub Actions examples, multilingual generation, immutable release catalogs, failure policy, and contract-versioning links.
• Add dedicated English and French logging-integration guides with a concrete structured event, required error properties, context handling, recursive InnerErrors, occurrence correlation, catalog links, log-level guidance, and sensitive-data rules.
• 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 examples were reviewed against the documented FirstClassErrors.Testing, Error, DiagnosableException, and catalog CLI APIs, and the cross-page navigation was 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. doc/README.fr.md itself did not require a change because the existing testing and operational-integration entries remain the entry points and now link to the new focused guides.

Related issues

None.