Add guidance for configuring nullable context to fix CS8632, CS8636, CS8637

[shethaadit]

Summary

This PR addresses issue #47305 by adding guidance to the "Configure nullable context" section in the nullable warnings documentation.

Changes Made

• Added detailed instructions on how to properly set the nullable context to resolve configuration warnings
• Included both project-level (<Nullable> element) and file-level (#nullable directive) configuration options
• Explained the two independent flags (annotation and warning) that control nullable behavior
• Added links to relevant documentation for nullable contexts and migration strategies

Why These Changes Are Needed

The original documentation listed the nullable context configuration warnings (CS8632, CS8636, CS8637) but didn't provide clear guidance on how to fix them. This enhancement helps developers quickly understand and resolve these configuration issues.

Related Issue

Fixes #47305

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/nullable-warnings.md	Resolve nullable warnings

[BillWagner]

Thanks @shethaadit

One small nit, and then this is ready.

[shethaadit]

Thanks @shethaadit

One small nit, and then this is ready.

Thank you @BillWagner. I have applied the suggestion. :)

[BillWagner]

This LGTM, once I fix the markdown lint issue