[ja] Japanese translation, First version

[hsugawa8651]

Japanese translation, First version

• Init Translation
  • [x] Choose a tag: ja
  • [x] add tag (ja) and translate the given phrases in _config.yml
  • [x] Create & Modify index.ja.html
  • [x] Copy the folder _includes/en/ to _includes/ja/
  • [x] Save an icon for ja to assets/ja.svg
  • [x] Add translated titles in file _data/blocks.yml
• Translation was carried out for
  • [x] Most of the contents in left columns
  • [x] Most of the sentences outside tables
• Not yet
  • [ ] Explanations in right columns

[brakmic]

Many thanks, @hsugawa8651 ! 👍