Clarify usage patterns and Outcome composition

[Reefact]

Summary

This PR separates usage decisions from the detailed Outcome API so readers can first choose the right error-propagation pattern, then learn fluent composition without mixing both concerns. It also clarifies how ErrorContext remains attached to an error across outcomes, exceptions, and inner-error composition.

Type of change

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

Changes

• Rewrite the English and French usage-pattern guides around a decision table and concrete domain, validation, batch, incoming-port, and outgoing-port examples.
• Add dedicated English and French guides for Outcome, Outcome<T>, Then, To, Recover, Finally, exception escape hatches, and asynchronous composition.
• Explain when fluent composition is clearer than ordinary branching and when it is not.
• Expand the English and French ErrorContext guides with clear role boundaries, transport-independent propagation, data-safety guidance, and a practical checklist.
• Correct the documentation to retrieve successful Outcome<T> results through GetResultOrThrow() rather than a non-existent Value property.
• 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 checked against the current Outcome, Outcome<T>, port-error, and context APIs, and 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 counterpart of every changed or added English page is included. doc/README.fr.md itself did not require a change because UsagePatterns.fr.md remains the existing entry point and now links to the dedicated Outcome guide.

Related issues

None.