[Docs] Add licensing header to C data interface in docs

### Describe the enhancement requested

At https://arrow.apache.org/docs/format/CDataInterface.html#structure-definitions, a header file for the C data interface is provided. It does not carry any direct license headers, but states:

> Like the rest of the Arrow project, they are available under the Apache License 2.0.

This can cause confusion for further redistribution and/or requires each user to correctly apply the corresponding header, while a recommended declaration could be provided by the Arrow project directly.

Depending on how you read/interpret the Apache-2.0 license in this regard, adding a license header myself could require me to add an explicit notice about me having added the license header (section 4b) and possibly even including the `NOTICE.txt` file of the Arrow project (section 4d).

### Component(s)

Documentation