docs: add project documentation

docs/DEVELOPMENT.md

@@ -0,0 +1,54 @@
+# Development
+
+## Prerequisites
+
+- Java 21
+- Node.js and npm compatible with `frontend/package.json`
+- Docker Engine and Docker Compose for container testing
+
+## Build and test
+
+From the repository root:
+
+```bash
+cd backend
+./gradlew clean build
+```
+
+This invokes the configured Gradle build, including frontend build tasks.
+
+## Frontend development
+
+Frontend source code is located in `frontend/`.
+
+Install dependencies using the lockfile:
+
+```bash
+cd frontend
+npm ci
+```
+
+Run the frontend development server using the project's configured npm scripts:
+
+```bash
+npm start
+```
+
+If the project does not define a `start` script, inspect `frontend/package.json` and use its available script, such as `npm run`.
+
+## Run the container
+
+From the repository root, after a successful build:
+
+```bash
+docker compose up -d --build
+docker compose logs -f htop-web
+```
+
+## Contribution guidelines
+
+- Keep changes focused and reviewable.
+- Do not commit generated build output or dependency directories.
+- Do not commit credentials, private keys, or local environment files.
+- Update documentation when build or runtime behavior changes.
+- Run the build before submitting changes.