docs: Add Japanese README

[gemmaro]

📑 Summary

This adds a Japanese translation of the readme and links to it.

There are no related issues.

📏 Design Decisions

This doesn't change implementation; document additions only.

The translation has been done with reference to the prior work of @NurseAngel as in README.ja-JP.md, although there are many changes.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests. (Skipped since there is no implementation change)
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• 🦋 If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:. (Checked that this is not the case)

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[pkg-pr-new]

Open in Stackblitz

pnpm add https://pkg.pr.new/mermaid-js/mermaid@6065

pnpm add https://pkg.pr.new/mermaid-js/mermaid/@mermaid-js/layout-elk@6065

pnpm add https://pkg.pr.new/mermaid-js/mermaid/@mermaid-js/mermaid-zenuml@6065

pnpm add https://pkg.pr.new/mermaid-js/mermaid/@mermaid-js/parser@6065

commit: a2a4865

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 4.49%. Comparing base (8328f74) to head (a2a4865).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #6065      +/-   ##
==========================================
- Coverage     4.49%   4.49%   -0.01%     
==========================================
  Files          382     383       +1     
  Lines        53914   53925      +11     
  Branches       621     596      -25     
==========================================
  Hits          2425    2425              
- Misses       51489   51500      +11     

Flag	Coverage Δ
unit	4.49% <ø> (-0.01%)	⬇️

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see 1 file with indirect coverage changes