Add Japanese version (hail2u#32)

* Create Japanese version

Issue maintained in: hail2u#29

* Translate section headings into Japanese

* Translate section headings of chapter 1–4

* Translate section headings of chapter 5

* Translate section headings of chapter 6

* Translate section headers of chapter 12

* Translate section headers of chapter 11

* Translate section headers of chapter 7–8

* Translate section headers of chapter 9

* Translate section headers of chapter 10

* Translate explanations of chapter 1

* Translate explanations of chapter 2

* Translate explanations of chapter 3

* Translate explanations of chapter 4

* Translate explanations of chapter 5

* Translate explanations of chapter 6

* Translate explanations of chapter 7

* Translate explanations of chapter 8

* Translate explanations of chapter 9

* Translate explanations of chapter 10

* Translate explanations of chapter 11

* Translate explanations of chapter 12