docs: complete documentation refactoring and structure optimization

This commit finalizes the documentation improvement plan by:
- Centralizing reference material in README.md.
- Creating a dedicated Technical Maintenance guide (docs/howtos/maintenance.md).
- Creating a categorized Sample Queries & Mutations overview (docs/queries/README.md).
- Eliminating redundant information across the doc set (DRY principle).
- Optimizing cross-references between reference documentation and the Cookbook.
- Updating the improvement plan to reflect all tasks as completed.

docs/howtos/maintenance.md

@@ -0,0 +1,58 @@
+# Technical Maintenance Guide
+
+This guide covers the technical aspects of maintaining and developing the Zabbix GraphQL API.
+
+## 🛠️ Development & Maintenance Tasks
+
+### Code Generation
+
+The project uses [GraphQL Codegen](https://the-guild.dev/graphql/codegen) to generate TypeScript types from the GraphQL schema. This ensures type safety and consistency between the schema and the implementation.
+
+- **Configuration**: `codegen.ts`
+- **Generated Output**: `src/schema/generated/graphql.ts`
+
+#### How to Regenerate Types
+Whenever you modify any `.graphql` files in the `schema/` directory, you must regenerate the TypeScript types:
+
+```bash
+npm run codegen
+```
+
+### Testing
+
+We use [Jest](https://jestjs.io/) for unit and integration testing.
+
+- **Test Directory**: `src/test/`
+- **Execution**:
+  ```bash
+  npm run test
+  ```
+
+#### Adding New Tests
+- **Location**: Place new test files in `src/test/` with the `.test.ts` extension.
+- **Coverage**: Ensure you cover both successful operations and error scenarios.
+- **Best Practice**: If you find a bug, first create a reproduction test in `src/test/` to verify the fix.
+
+## 🔄 Updating Dependencies
+
+To keep the project secure and up-to-date:
+
+1. Check for updates: `npm outdated`
+2. Update packages: `npm update`
+3. Verify with tests: `npm run test`
+
+## 🐳 Docker Image Maintenance
+
+The `Dockerfile` uses a multi-stage build to keep the production image small and secure.
+
+- **Base Image**: Node 24 (LTS)
+- **Build Argument**: `API_VERSION` (used to embed the version into the image)
+
+To build a fresh image locally:
+```bash
+docker build -t zabbix-graphql-api --build-arg API_VERSION=$(git describe --tags --always) .
+```
+
+---
+**Related Reference**: [Project Configuration Reference](../../README.md#configuration)
+**Related Cookbook**: [Extending the Schema](./cookbook.md#recipe-extending-schema-with-a-new-device-type)