Add Japanese translations and implement I18N support

[yostos]

This pull request adds Japanese translations for the documentation and implements I18N support for the project.

Intended changes:

• Add Japanese translations for main documentation files
• Implement an I18N framework to support multiple languages
• Update mkdocs.yml to enable multi-language support

Motivation:
To make the project more accessible to Japanese-speaking users and to establish a framework for easy addition of other languages in the future.

There are no directly related issues, but this change aligns with the project's goal of international accessibility.

Example of changes:

• New 'ja/' directory added with translated .md files
• mkdocs.yml updated to include language selection options

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[wren]

Hi, @yostos! Wow! This seems like it must have been a lot of work, so thank you!

A few notes:

• Can you remove the auto-generated files (pretty much everything inside of the site/ folder)?
• It seems like you might have an auto-formatter on. Can you please keep the PR focused on the relevant changes (example: there are a lot of unrelated changes inside of pyproject.toml file)

Does that make sense? And please let us know if you need help with any of these things.