Update Quickstart of ja/docs according to en/docs

[lef237]

There was no description of go install in the Japanese documentation.

It exists in the English documentation, so I have updated it.

https://github.com/gin-gonic/website/blob/437102130e9320f0184a956a9e58f805e17dbdfa/content/en/docs/quickstart/_index.md?plain=1#L22-L25

Best regards

[lef237]

I confirmed that the code does not work properly with go install.

I realized that I should use go mod init example.com/m/v2 and go mod tidy, so I close this PR.