docs: add comprehensive multi-language documentation (en-US + pt-BR)

[devin-ai-integration]

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Summary

Complete multi-language documentation overhaul for the metar-decoder repository.

Documentation Changes

README files:

• Rewrote README.md in English (en-US) as the default language
• Created README.pt-br.md with full Portuguese (pt-BR) translation

Detailed guides (docs/ folder):

• docs/en-US/usage-guide.md — Comprehensive usage guide with METAR and TAF examples, unit conversion, parsing modes, error handling
• docs/en-US/api-reference.md — Complete API reference for all public classes, properties, methods, and enumerations
• docs/pt-BR/guia-de-uso.md — Portuguese usage guide
• docs/pt-BR/referencia-api.md — Portuguese API reference

NuGet package READMEs (en-US only):

• src/Metar.Decoder/Assets/nuget-readme.md — Metar.Decoder package README with quick start, features, unit conversion examples, and repository links
• src/Taf.Decoder/Assets/nuget-readme.md — Taf.Decoder package README with quick start, features, evolution examples, and repository links
• Updated both .csproj files to reference the new NuGet readme files

XML documentation improvements:

• Improved class-level summaries on SurfaceWind, Visibility, WeatherPhenomenon, Value
• Added missing Cavok summary on DecodedMetar and DecodedTaf
• Fixed typo in Temperature.cs ("Hur" → "Hour")
• Improved method documentation on SetDirectionVariations, AddType
• Enhanced Value.Unit enum documentation

Build

• dotnet build MetarDecoder.sln --configuration Release passes successfully

Link to Devin session: https://app.devin.ai/sessions/e7852e5f759e4ba9a2ed414a0f8cc7e6
Requested by: @afonsoft

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[afonsoft]

✅ Snyk checks have passed. No issues have been found so far.

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[sonarqubecloud]

Quality Gate failed

Failed conditions
19.2% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud