Update task summary 
This is the second part of updating the task summary to be more conceptual. After a brief introduction and background to the tasks Transformers can solve in [part 1](https://github.com/huggingface/transformers/pull/21014), this PR is a bit more advanced and digs deeper into explaining how Transformer solves these tasks.

### To-do:
- [x] Add computer vision section
- [x] Add NLP section
_The documentation is not available anymore as the PR was closed or merged._<|||||>Ok, I'm finally finished with the first draft (took a bit longer to learn some models I wasn't familiar with)! I'd appreciate a general review of the scope of this page to make sure we're aligned (ie, are some sections too in-depth, are some not explained well enough?). Thanks in advance @sgugger @MKhalusova ! 🥹

Afterward, I'll ping one of our audio and computer vision experts for a more in-depth review of those sections 🙂 <|||||>Thanks for the feedback, I added some images to go along with the text!

@NielsRogge, would you mind reviewing the computer vision section? This guide is a high-level overview, and the goal is to help users understand how a certain task is solved by a model. Please feel free to let me know if it's too detailed, not detailed enough, or if I got something wrong! Also, if you know of a good beginner's resource for computer vision we can link to, that'd be great as well to set expectations for the reader. Thanks! 👍

@sanchit-gandhi, if you could do the same with the audio section, that'd be awesome. Thank you! 👍