How to document external file in doxygen?

@domenico 

To document an external file in Doxygen, you can use the ile command followed by a brief description of the file. Here is how you can document an external file in Doxygen:

1. Place a comment at the beginning of the external file you want to document. Start the comment with /*! or /** to indicate a Doxygen comment block.
2. Use the ile command followed by the name of the file within angle brackets, like this: ile
3. Provide a brief description of the file after the ile command.
4. Optionally, you can also include other Doxygen commands to provide more detailed documentation about the contents of the file, such as rief, author, date, and so on.

Here is an example of how to document an external file in Doxygen:

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/*!
 * ile external_file.h
 * rief This file contains the declaration of the ExternalClass class.
 *
 * author John Doe
 * date July 2021
 */

class ExternalClass {
  // class definition...
};

After documenting the external file, you can generate the documentation using Doxygen, and the external file will be included in the generated documentation along with the specified description and other information.