Documentation of custom_translations_url in config.json

[Justinzobel]

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