docs: add federation + Prisma usage guidance

[macrozone]

Summary

• Documents common issue with field selection in resolveReference when using federation plugin with Prisma
• Exposed fields (t.exposeString(), t.exposeInt(), etc.) are not automatically included in select, causing reference resolution failures across subgraphs
• Provides solution using queryFromInfo to automatically determine required fields based on GraphQL query information

Changes

• Added new "Usage with Prisma" section to federation plugin documentation
• Includes problem example showing the issue
• Provides recommended solution using queryFromInfo from @pothos/plugin-prisma

Test plan

• Documentation is clear and follows existing patterns
• Code examples are syntactically correct
• Solution addresses the reported issue

Fixes #1321

🤖 Generated with Claude Code

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