Documentation of custom_translations_url in config.json

### Your use case

#### What would you like to do?
Add instructions on how to use custom_translations_url in config.json

#### Why would you like to do it?
Because there are no docs on how to use it this and I've tried several different methods

#### How would you like to achieve it?
Add to https://github.com/element-hq/element-web/blob/develop/docs/config.md
Currently it just sends users to a Pull Request which doesn't help at all.
A sample config with custom_translations_url enabled and a sample file to show how the layout should be.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_