Add C# MCP Server Development collection (#323)

* Add C# MCP Server Development collection

- Add instructions for C# MCP server development best practices
- Add prompt for generating complete MCP server projects
- Add expert chat mode for C# MCP development assistance
- Add collection grouping all C# MCP resources together
- Update README files with new contributions

* Add applyTo field to C# MCP instructions

collections/csharp-mcp-development.collection.yml

@@ -0,0 +1,32 @@
+id: csharp-mcp-development
+name: C# MCP Server Development
+description: Complete toolkit for building Model Context Protocol (MCP) servers in C# using the official SDK. Includes instructions for best practices, a prompt for generating servers, and an expert chat mode for guidance.
+tags: [csharp, mcp, model-context-protocol, dotnet, server-development]
+items:
+  - path: instructions/csharp-mcp-server.instructions.md
+    kind: instruction
+  - path: prompts/csharp-mcp-server-generator.prompt.md
+    kind: prompt
+  - path: chatmodes/csharp-mcp-expert.chatmode.md
+    kind: chat-mode
+    usage: |
+      recommended
+
+      This chat mode provides expert guidance for building MCP servers in C#.
+
+      This chat mode is ideal for:
+      - Creating new MCP server projects
+      - Implementing tools and prompts
+      - Debugging protocol issues
+      - Optimizing server performance
+      - Learning MCP best practices
+
+      To get the best results, consider:
+      - Using the instruction file to set context for all Copilot interactions
+      - Using the prompt to generate initial project structure
+      - Switching to the expert chat mode for detailed implementation help
+      - Providing specific details about what tools or functionality you need
+
+display:
+  ordering: manual
+  show_badge: true