Document Creating Multiple Attachments Using A Web Browser

[NOTE]: # ( ^^ Provide a general summary of the request in the title above. ^^ )

## Summary

[The documentation](https://docs.couchdb.org/en/stable/api/document/common.html#creating-multiple-attachments) shows that it is possible to load multiple attachments in a single HTTP PUT, the `Content-Type` being `multipart/related`. Web browser methods XMLHTTPRequest and Fetch do not natively produce HTTP requests with this `Content-Type`.

## Desired Behaviour

Publish JavaScript illustrating how to PUT multiple attachments in a single request to CouchDB.

NOTE: I tried to proxy a request crafted with `FormData()` through Node.js and simply rewrite the `Content-Type` (keeping the `boundary`) but this fails with a `500` error.

Start with the existing “Creating multiple attachments” documentation section and review the multipart/related requirements described there, then compare them with browser XMLHttpRequest, Fetch, and FormData behavior. Done means the CouchDB documentation includes a working JavaScript browser example for PUTting multiple attachments and explains the required request format.

Written by the indexing model from the issue text.