docs: add example repository and update documentation

[Yukaii]

This pull request introduces a comprehensive example project for the HackMD API client and modernizes the codebase by transitioning to ES Modules (ESM) and adding support for both ESM and CommonJS builds. It also enhances the API client's features with retry logic, ETag-based caching, and improved error handling. Below are the most significant changes grouped by theme:

Example Project for HackMD API Client

• Added a complete example project (examples/nodejs/) demonstrating key API features such as user information retrieval, note creation, content updates, and error handling. This includes a detailed README.md with setup instructions and usage examples. [1] [2]
• Introduced an .env.example file to guide users in setting up environment variables for the example project.

Transition to ES Modules and Build System Modernization

• Updated nodejs/package.json to use ESM by default, added exports field for dual ESM and CommonJS compatibility, and replaced tsc with Rollup for building the library. [1] [2]
• Added a rollup.config.js file to configure ESM and CommonJS builds with source maps and external dependencies.
• Updated tsconfig.base.json to use ESNext for both module and target, aligning with modern JavaScript standards.

API Client Enhancements

• Introduced retry logic with exponential backoff for failed requests, ETag-based caching for note retrieval, and options for raw response handling in the API client.
• Exported all error classes (HackMDError, HttpResponseError, etc.) individually to improve modularity and usability. [1] [2] [3]

Documentation Improvements

• Expanded the main README.md with examples for ESM, CommonJS, and legacy import paths, as well as advanced features like retry configuration and response data handling.
• Added a new "Examples" section to the repository's README.md to highlight the example project and its features.

Miscellaneous Updates

• Renamed nodejs/.eslintrc.js to nodejs/.eslintrc.cjs to align with ESM conventions.
• Updated package-lock.json and dependencies to reflect the new build system and added required plugins for Rollup. [1] [2]