Formats mapping and removes first two levels

I wasn't able to load the mapping unless I removed the first two levels (`congress` and `mappings`) -- it looks like those were attempting to update all mappings in the `congress` index at once.

This mapping begins at the collection-level:

```json
{
  "congressional_documents": {
    "properties": {
      "bill_id": {
        "type": "string"
      },
  ...
}
```

And is in line with the other mappings in `config/mappings/`.

I also ran the JSON through a JSON formatter to normalize the tabs to be more readable, and to match the other mappings.