Fix OpenAI key error. Add docstrings. Polish code

[dSupertramp]

Hi everyone!
This tool is amazing! I really want to be part of it!

Here's some changes:

• Add python-dotenv, to make sure that the OpenAI API key works
• Add some docstrings and some typing
• Add python-dotenv into Pipfile

I'm willing to work into the extraction part!

[chloedia]

Hey @dSupertramp thanks for contributing ! Don't hesitate to suggest ideas on the extraction part.

On my end I think it would be interesting to try upgrading the table integration and adding other medias like images for pdf ! With a notion of :
-> is the image really relevant to be added in the md
-> add image description in img metadata

Using multimodal !
What do you think ?

There is also a lot to add on the docx/pptx part to have the cleanest md possible!