docs: update CONTRIBUTING.md for IntelliJ and JDK 17 setup (Fixes #405)

Author: rexminnis

CONTRIBUTING.md

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+# Contributing to RayDP
+
+Thank you for your interest in contributing to RayDP!
+
+## Development Environment Setup
+
+RayDP is a mixed-language project (Scala + Python). To set up your development environment correctly in IntelliJ IDEA, please follow these steps.
+
+### Prerequisites
+* **Java:** JDK 17 (Recommended to match CI).
+* **Python:** Python 3.8+ (Recommended: Use Conda or virtualenv).
+* **Maven:** 3.6+
+
+### IntelliJ IDEA Configuration
+
+1.  **Import the Project**
+    * Open the root `raydp` folder in IntelliJ.
+    * If prompted, choose **"Trust Project"**.
+    * If the Maven project is not auto-detected, right-click `core/pom.xml` in the Project view and select **"Add as Maven Project"**.
+
+2.  **Project Structure** (`Cmd + ;` on macOS)
+    * Go to **Project Settings > Project**.
+    * Set **SDK** to **17** (Java 17).
+    * Set **Language Level** to **17**.
+
+3.  **Module Configuration**
+    To ensure code completion works for both Scala and Python:
+    * Go to **Project Settings > Modules**.
+    * **Scala:** Select the `raydp-core` module. Ensure `src/main/scala` is marked as **Sources** (Blue) and `src/test/scala` as **Tests** (Green).
+    * **Python:** Click `+` > **New Module** (or "Import Module").
+        * Select the `python/` directory in your project root.
+        * Set the **Module SDK** to your local Python environment (e.g., Conda `raydp-dev`).
+        * Mark `python/raydp` as **Sources** if not automatically detected.
+
+### Verification
+To verify your setup, run the build from the terminal (or the Maven side panel):
+
+```bash
+mvn clean package -DskipTests