[ja] Translate content/en/docs/tasks/extend-kubernetes/custom-resources/_index.md into Japanese

**This is a Feature Request**

**What would you like to be added**

Translate `content/en/docs/tasks/extend-kubernetes/custom-resources/_index.md` into Japanese

**Website Link**

- English: https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/

**Why is this needed**

This page is not translated yet.

/area localization
/language ja
/assign