Add answer key for C# Basics workshop

[beagandica]

Summary

Add a comprehensive answer key for the C# Basics workshop covering all interactive challenges.

Changes

New file: \content/english/csharp-basics/answer-key.md\

Solutions for all 5 challenge groups:

1. Variables - Help Patrick - Fix data type mismatches (string, int, double, char, bool) with explanation table
2. Operators - Exam Statistics - Declare grade variables, calculate class average, use relational operators
3. Control Structures - If-Statement - Boolean expressions for Patrick's greeting (morning/afternoon/night)
4. Control Structures - For-Loop - Convert while loop to for loop (sum 1-100)
5. Methods - Pyramid Count - pyramidCount method using sum of squares
6. Classes and Objects - Bird Class - Complete class with fields, constructor, getters, and ToString

Each solution includes:

• The code solution
• Expected output
• Kid-friendly explanation of how it works

Solutions verified against the actual .NET Fiddle starter code (Widgets xKMKvn, mM7xbj, T6AUdh, ireaAA, OH5XQO).

Languages affected

• English (primary)
• Brazilian Portuguese has the same challenges (same .NET Fiddle widgets with identical C# code) — no separate answer key needed since solutions are code-based

Testing

• Hugo build passes
• All language versions checked (EN + PT-BR)
• 10-model QA passed (3 clean passes)
• Follows python-basics answer key format (PR Add answer key for python-basics workshop (26 exercises, 5 sections) #633)
• Hidden from navigation (\hidden: true, \weight: 15)