add mutation conventions

[danstarns]

Was going back and forth wondering why my function was not being exposed via the graphql mutations api, turns out its a hardcoded convention in the codebase located here:

• https://github.com/hypermodeinc/modus/blob/main/runtime/graphql/schemagen/conventions.go

[Copilot]

Pull Request Overview

This PR adds documentation for GraphQL mutation naming conventions, outlining how exported functions are classified based on their name prefixes.

• Introduces a new Generating Mutations section in the API guide.
• Lists the exact prefixes that trigger mutation generation.
• Provides examples contrasting query vs. mutation naming.

Comments suppressed due to low confidence (1)

modus/api-generation.mdx:146

• Clarify whether the prefix matching is case-sensitive or case-insensitive so users know if variants like AddUser vs addUser are both recognized as mutations.

Functions are automatically classified as **mutations** when they start with these prefixes:

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