docs(ja): add Japanese docs for vuex 4.0

[ryo-gk]

This PR adds Japanese docs for vuex v4.0.

TODO

Translation

• docs/ja/api/index.md
• docs/ja/guide/actions.md
• docs/ja/guide/composition-api.md
• docs/ja/guide/forms.md
• docs/ja/guide/getters.md
• docs/ja/guide/hot-reload.md
• docs/ja/guide/index.md
• docs/ja/guide/migrating-to-4-0-from-3-x.md
• docs/ja/guide/modules.md
• docs/ja/guide/mutations.md
• docs/ja/guide/plugins.md
• docs/ja/guide/state.md
• docs/ja/guide/strict.md
• docs/ja/guide/structure.md
• docs/ja/guide/testing.md
• docs/ja/guide/typescript-support.md
• docs/ja/index.md
• docs/ja/installation.md

[ryo-gk]

All translation is done!
Could you review it?

For your reference,

• The target files follow the zh translation PR (docs: add zh docs for vuex 4.0 #1965).
• The parts that have not been changed between 3.0 and 4.0 of original docs have been translated using the same Japanese as in 3.0.

[kiaking]

Wonderful job!!! Thank you so much 🙏 🙏 🙏