[docs, help wanted] Frontend Usage Examples

We often see Issues opened for folks who are having troubles on the client side. Uploading an array of files clientside is non-obvious, due to no fault of Multer.

Most recently #880, but there are many such examples. (@expressjs/triagers maybe link some related issues here if you find them?)

PR https://github.com/expressjs/multer/pull/580 addressed this with a regular HTML form.

I propose a specific section of the docs, something like ## Clientside Examples to feature prominently, and allow easy linking to, with common examples.

Specifically we are missing an example for FormData web api. We can't cover everything, but let's cover some of the most asked cases.