Add Japanese translation of "GETTING_STARTED"

[raisa0726]

Problem this feature will solve

I do not have enough knowledge to use FactoryBot. Also, some of my colleagues find it difficult to read English documentation.

Desired solution

Help Japanese people understand FactoryBot more easily.

Alternatives considered

Ask for approval to prepare and open a Japanese translation of GETTING_STARTED.

Additional context

Japanese translation of GETTING_STARTED was created by me in Qiita. Here it is tentatively available to share the source.