Add Japanese translations

[shookietea]

It appears that mkdocs-material allows us to specify multiple languages. This adds a button at the top for language selection, which will render pages from a specific subdirectory.

https://squidfunk.github.io/mkdocs-material/setup/changing-the-language/

Specifically:

• Move English notes under /en
• Move any Japanese under /ja
• The notes format and content should match exactly for now

[shookietea]

It may be beneficial to not split Level notes into separate English / Japanese pages. Since these are mainly composed of tables, a Japanese language notes column may be sufficient.

[shookietea]

https://docs.weblate.org/ - seems like a cool non-free project localization tool. https://www.mkdocs.org/dev-guide/translations/ - will probably be the route to take.