Add embedded HTML index page and serve it in response

This commit introduces an embedded `index.html` file to the project using the `//go:embed` directive. It also updates the server to set the appropriate content type and serve the embedded HTML page as the response for HTTP requests. This enhances the project by adding a user-facing interface.

Set target platforms for Docker image builds

Added `linux/amd64` and `linux/arm64` to the Docker build platforms in the GitHub Actions workflow. This ensures multi-platform support for the resulting Docker images, improving compatibility across different systems.