Update task summary part 1
This PR reworks the task summary to be more conceptual and provides more explanation about a topic to help users better understand it. It'll be focused more on understanding instead of practical steps. The update will be split into two parts:

1. Describe the tasks 🤗 Transformers is capable of solving (the focus of this PR). Provide some context about how these tasks used to be solved, how they're handled now, and practical applications of each task.
2. Explain how 🤗 Transformers solve these tasks. This'll be a more conceptually advanced and separate page.
_The documentation is not available anymore as the PR was closed or merged._<|||||>> It is called "what Transformers can do", so I think the introductions of each modality should just explain what each modality is and abstain from comparing models

Thanks for the feedback, this helped me refine the scope of this page! It was a little difficult trying to discuss the tasks and not be tempted to also talk about the models since the two are so closely related. 😅 I updated the intro of each modality with an explanation of the input data and how to get it into a useable format by the model to solve a task.