Add code documentation for `process()`

[francisohara24]

I added docstrings and type annotations for the process() function to make it easier for users to figure out how exactly the package can be used to extract text and images. Although documentation is provided in the README.md file in the package's GitHub repository, I think it's important that the code itself is documented as well so that users don't have to visit the repository to figure out how to use the package, which is the situation I found myself in. It'll also help IDEs do code completion for developers using the package.