docs: add contributing-plugins guide to docs-v0.56.x

docs/content/en/docs-v0.56.x/contribution-guidelines/contributing-plugins/_index.md

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+---
+title: "Contribute to PipeCD plugins"
+linkTitle: "Contribute to PipeCD plugins"
+weight: 4
+description: >
+  Learn how to create PipeCD plugins.
+---
+
+PipeCD's plugin architecture allows anyone to extend `piped`'s 
+capabilities by creating custom plugins. This guide explains how 
+to develop and contribute plugins.
+
+## Understanding the plugin architecture
+
+In PipeCD v1, plugins are the actors that execute deployments on 
+behalf of `piped`. Instead of `piped` directly deploying to platforms, 
+plugins handle platform-specific logic while `piped`'s core controls 
+deployment flows.
+
+**Key concepts:**
+- **Plugins** run as gRPC servers, launched and managed by `piped`
+- **Deploy targets** define where a plugin deploys (e.g., a Kubernetes cluster)
+- Plugins can be **official** (maintained by PipeCD team) or **community-contributed**
+
+For a detailed overview, see the [Plugin Architecture blog post](https://pipecd.dev/blog/2024/11/28/overview-of-the-plan-for-pluginnable-pipecd/).
+
+## Plugin types
+
+Plugins can implement one or more of these interfaces:
+
+| Interface | Purpose |
+|-----------|---------|
+| **Deployment** | Plan and execute deployment stages |
+| **LiveState** | Fetch and build the state of live resources |
+| **Drift** | Calculate drift between live and git-source manifests |
+
+## Where plugins live
+
+- **Official plugins**: Located in `/pkg/app/pipedv1/plugin/` in the [pipecd repository](https://github.com/pipe-cd/pipecd)
+- **Community plugins**: Located in the [pipe-cd/community-plugins](https://github.com/pipe-cd/community-plugins) repository
+
+## Getting started
+
+### Prerequisites
+
+- [Go 1.24 or later](https://go.dev/)
+- Understanding of gRPC
+- Familiarity with the platform you're building a plugin for
+
+### Study existing plugins
+
+| Plugin | Complexity | Good for learning |
+|--------|------------|-------------------|
+| [wait](https://github.com/pipe-cd/pipecd/tree/master/pkg/app/pipedv1/plugin/wait) | Simple | Basic plugin structure |
+| [waitapproval](https://github.com/pipe-cd/pipecd/tree/master/pkg/app/pipedv1/plugin/waitapproval) | Simple | Stage-only plugin |
+| [kubernetes](https://github.com/pipe-cd/pipecd/tree/master/pkg/app/pipedv1/plugin/kubernetes) | Complex | Full-featured plugin |
+| [terraform](https://github.com/pipe-cd/pipecd/tree/master/pkg/app/pipedv1/plugin/terraform) | Complex | Infrastructure as Code plugin |
+
+### Plugin structure
+your-plugin/
+├── go.mod
+├── go.sum
+├── main.go
+├── plugin.go
+├── config/
+│   └── application.go
+└── README.md
+
+## Contributing to official plugins
+
+1. **Open an issue** first to discuss your plugin idea with maintainers
+2. **Fork and clone** the [pipecd repository](https://github.com/pipe-cd/pipecd)
+3. **Create your plugin** under `/pkg/app/pipedv1/plugin/your-plugin/`
+4. **Write tests**: see existing plugins for patterns
+5. **Add a README** documenting configuration and usage
+6. **Submit a PR** linking to the discussion issue
+
+### Build and test
+
+```bash
+# Build all plugins
+make build/plugin
+
+# Run tests
+make test/go
+
+# Run piped locally with your plugin
+make run/piped CONFIG_FILE=piped-config.yaml EXPERIMENTAL=true INSECURE=true
+```
+
+## Contributing to community plugins
+
+The [community-plugins repository](https://github.com/pipe-cd/community-plugins) 
+welcomes plugins that may not fit in the official repo.
+
+1. **Fork** the community-plugins repository
+2. **Create your plugin** following the structure above
+3. **Submit a PR** with documentation
+
+## Resources
+
+- [Plugin Architecture RFC](https://github.com/pipe-cd/pipecd/blob/master/docs/rfcs/0015-pipecd-plugin-arch-meta.md)
+- [Plugin Architecture blog post](https://pipecd.dev/blog/2024/11/28/overview-of-the-plan-for-pluginnable-pipecd/)
+- [Plugin Alpha Release blog](https://pipecd.dev/blog/2025/06/16/plugin-architecture-piped-alpha-version-has-been-released/)
+- [#pipecd Slack channel](https://cloud-native.slack.com/) for questions
+
+Thank you for contributing to PipeCD plugins!