[ja] Update content/ja/docs/concepts/_index.md

[kfess]

This is a Feature Request

What would you like to be added

Update the Japanese translation of the following page to reflect the latest English version:

• Japanese: https://kubernetes.io/ja/docs/concepts/
• English: https://kubernetes.io/docs/concepts/

Why is this needed

The current Japanese translation is outdated.

[kfess]

/language ja
/assign

[Okabe-Junya]

/area localization
/triage accepted