When reading from a `Stream`, like `File`, using a temporary buffer is counterproductive because it complexifies the code and increases memory usage.
It's also a source of confusion because it can create dangling pointers in the `JsonDocument`.
The only benefit of using a buffer is the reading speed, but I don't think speed is the focus in this example; if it were, it would use a buffer in `saveConfig()` too.
612e7ffd7f