Add warnings and guidance against using volatile keyword

[Copilot]

Fixes #23832

The volatile keyword documentation now includes prominent warnings about its limitations and strongly recommends safer alternatives for multithreaded programming.

Changes Made

• Added prominent CAUTION block warning against misuse of volatile early in the document
• Created new "Alternatives to volatile" section explaining safer options:
  • System.Threading.Interlocked operations for atomic operations
  • lock statements for mutual exclusion
  • System.Threading.Volatile class for explicit volatile operations
  • Higher-level synchronization primitives and concurrent collections
• Enhanced existing content to emphasize safer alternatives throughout the document
• Expanded "See also" section with modern threading resources

Why These Changes

The C# community has identified that volatile is often misunderstood and misused, leading to subtle concurrency bugs. The existing documentation provided technical information but lacked guidance about when not to use volatile and what to use instead.

What This Addresses

• Warns developers about common misconceptions with volatile
• Explains that volatile doesn't provide atomicity, prevent race conditions, or guarantee memory ordering
• Directs developers toward modern, safer concurrency tools
• Maintains all existing technical content for those who need to understand volatile

The documentation now follows the community consensus that volatile should be avoided in favor of more reliable alternatives while still providing the technical details for advanced scenarios.

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[BillWagner]

@copilot Can you start working on this again?