Copy contribute/style/content-guide.md from en/ directory.

Author: shuuji3

content/ja/docs/contribute/style/content-guide.md

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+---
+title: Documentation Content Guide
+linktitle: Content guide
+content_type: concept
+weight: 10
+---
+
+<!-- overview -->
+
+This page contains guidelines for Kubernetes documentation.
+
+If you have questions about what's allowed, join the #sig-docs channel in
+[Kubernetes Slack](https://slack.k8s.io/) and ask!
+
+You can register for Kubernetes Slack at https://slack.k8s.io/.
+
+For information on creating new content for the Kubernetes
+docs, follow the [style guide](/docs/contribute/style/style-guide).
+
+
+
+<!-- body -->
+
+## Overview
+
+Source for the Kubernetes website, including the docs, resides in the
+[kubernetes/website](https://github.com/kubernetes/website) repository.
+
+Located in the `kubernetes/website/content/<language_code>/docs` folder, the
+majority of Kubernetes documentation is specific to the [Kubernetes
+project](https://github.com/kubernetes/kubernetes).
+
+## What's allowed
+
+Kubernetes docs allow content for third-party projects only when:
+
+- Content documents software in the Kubernetes project
+- Content documents software that's out of project but necessary for Kubernetes to function
+- Content is canonical on kubernetes.io, or links to canonical content elsewhere
+
+### Third party content
+
+Kubernetes documentation includes applied examples of projects in the Kubernetes project&mdash;projects that live in the [kubernetes](https://github.com/kubernetes) and
+[kubernetes-sigs](https://github.com/kubernetes-sigs) GitHub organizations.
+
+Links to active content in the Kubernetes project are always allowed.
+
+Kubernetes requires some third party content to function. Examples include container runtimes (containerd, CRI-O, Docker),
+[networking policy](/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/) (CNI plugins), [Ingress controllers](/docs/concepts/services-networking/ingress-controllers/), and [logging](/docs/concepts/cluster-administration/logging/).
+
+Docs can link to third-party open source software (OSS) outside the Kubernetes project only if it's necessary for Kubernetes to function.
+
+### Dual sourced content
+
+Wherever possible, Kubernetes docs link to canonical sources instead of hosting
+dual-sourced content.
+
+Dual-sourced content requires double the effort (or more!) to maintain
+and grows stale more quickly.
+
+{{< note >}}
+
+If you're a maintainer for a Kubernetes project and need help hosting your own docs,
+ask for help in [#sig-docs on Kubernetes Slack](https://kubernetes.slack.com/messages/C1J0BPD2M/).
+
+{{< /note >}}
+
+### More information
+
+If you have questions about allowed content, join the [Kubernetes Slack](https://slack.k8s.io/) #sig-docs channel and ask!
+
+
+
+## {{% heading "whatsnext" %}}
+
+
+* Read the [Style guide](/docs/contribute/style/style-guide).