chore(i18n,docs): initialise japanese docs (#43527)

docs/i18n/japanese/index.md

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+The [freeCodeCamp.org](https://freecodecamp.org) community is possible thanks to thousands of kind volunteers like you. If you want to contribute your time and expertise, we would be excited to welcome you aboard.
+
+> [!NOTE] Before you proceed, please take a quick 2 minutes to read our [Code of Conduct](https://www.freecodecamp.org/code-of-conduct). We strictly enforce it across our community so that contributing to freeCodeCamp.org is a safe, inclusive experience for everyone.
+
+Happy contributing.
+
+You are welcome to:
+
+- Create, update and fix bugs in our [curriculum](#curriculum).
+- Help us fix bugs in freeCodeCamp.org's [learning platform](#learning-platform).
+- [Help us translate](#translations) freeCodeCamp.org to world languages.
+
+We answer the most common questions about contributing [in our contributor FAQ](FAQ.md).
+
+## Curriculum
+
+Our curriculum is curated by the global freeCodeCamp community. This way, we are able to incorporate expert knowledge from volunteers like you.
+
+You can help expand and improve the curriculum. You can also update project user stories to better-explain concepts. And you can improve our automated tests so that we can more accurately test people's code.
+
+**If you're interested in improving our curriculum, here's [how to contribute to the curriculum](how-to-work-on-coding-challenges.md).**
+
+## Translations
+
+We are localizing freeCodeCamp.org to major world languages. Some of the certifications are already live in [Chinese (中文)](https://chinese.freecodecamp.org/learn) and [Spanish (Español)](https://www.freecodecamp.org/espanol/learn/). We encourage you to read the [announcement here](https://www.freecodecamp.org/news/world-language-translation-effort) and share it your friends to get them excited about this.
+
+**If you're interested in translating, here's [how to translate freeCodeCamp's resources](how-to-translate-files.md).**
+
+## Learning Platform
+
+Our learning platform runs on a modern JavaScript stack. It has various components, tools, and libraries. These include Node.js, MongoDB, OAuth 2.0, React, Gatsby, Webpack, and more.
+
+Broadly, we use
+
+- a Node.js based API server
+- a set of React-based client applications
+- and testing scripts to evaluate camper-submitted curriculum projects.
+
+If you want to productively contribute to the learning platform, we recommend some familiarity with these tools.
+
+If you want to help us improve our codebase...
+
+**you can either use Gitpod, a free online dev environment that starts a ready-to-code dev environment for freeCodeCamp in your browser.**
+
+[![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/freeCodeCamp/freeCodeCamp)
+
+Or you can...
+
+**[set up freeCodeCamp locally](how-to-setup-freecodecamp-locally.md) on your machine.**