Translate concepts/extend-kubernetes/extend-cluster/ into Japanese

**This is a...**
- [x] Feature Request
- [ ] Bug Report

**Problem:**
No `concepts/concepts/extend-kubernetes/extend-cluster.md` in Japanese.

**Proposed Solution:**
Copy a file from en contents to `content/ja/docs/concepts/extend-kubernetes/extend-cluster.md` and translate it into Japanese.

**Page to Update:**
https://kubernetes.io/ja/docs/concepts/extend-kubernetes/extend-cluster/