Add techniques/methods for uploading & downloading multiple files in Cloud Storage

### [REQUIRED] Describe your environment

* Firebase Product: Cloud Storage
* Operating System version, Browser version, Firebase SDK version: n/a

### [REQUIRED] Describe the problem

Documentation currently only shows how to upload and download single files from Cloud storage. Due to the unique and particular nature of working with files, I think it would be great to include strategies and methods showing how to upload and download multiple files at once.

It's a pretty common action in many real-world applications.

While some answers can be found on Stack Overflow, it would be much easier to have proven solutions with more relevant context showcased in the docs.