fix: replace broken include path with actual Go code content

[Copilot]

The mdBook build was failing due to a broken include directive in the plugin documentation. The file was trying to include Go code from a path that doesn't exist in this repository:

{{#include ./../../../../../pkg/plugins/golang/v4/scaffolds/internal/templates/gomod.go}}

This caused build errors:

[ERROR] (mdbook::preprocess::links): Error updating "{{#include ./../../../../../pkg/plugins/golang/v4/scaffolds/internal/templates/gomod.go}}", Could not read file for link {{#include ./../../../../../pkg/plugins/golang/v4/scaffolds/internal/templates/gomod.go}} (/home/runner/work/kubebuilder-zh-docs/kubebuilder-zh-docs/book/src/plugins/extending/./../../../../../pkg/plugins/golang/v4/scaffolds/internal/templates/gomod.go)
[WARN] (mdbook::preprocess::links): Caused By: No such file or directory (os error 2)

Solution:
Replaced the broken include directive with the actual Go code content from the kubebuilder repository's pkg/plugins/golang/v4/scaffolds/internal/templates/gomod.go file. This ensures the documentation displays the correct example code for implementing machinery interface objects in custom plugins.

Result:

• ✅ Build completes successfully without errors
• ✅ Documentation renders correctly with proper Go syntax highlighting
• ✅ Example code is now visible to readers learning about Kubebuilder plugin development

Fixes #1.

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