New documentation structure
I just did some minor changes + added some content in these sections: share, about arrow, about cache

Feel free to mark this PR as ready for review ! :) I just separated the `Share` How-to page into three pages: share, dataset_script and dataset_card.

This way in the share page we can explain in more details how to share a community or a canonical dataset - focus in their differences and the steps to upload them.

Also given that making a dataset script or a dataset card both require several steps, I feel like it's better to have dedicated pages for them.

Let me know what you think @stevhliu and others. We can still revert this change if you feel like it was better with everything in the same place. I just added some minor changes to match the style, fix typos, etc. Great work on the conceptual guides, I learned a lot from them and I'm sure they will help a lot of other people too!

I am fine with splitting `Share` into three separate pages. I think this probably makes it easier for users to navigate, instead of having to scroll up and down on a really long single page. Thanks a lot for all the suggestions ! I'm doing the final changes based on the remaining comments, then we can merge and release v1.12 of `datasets` and the new documentation ^^ Alright I think I took all the suggestions and comments into account :)
Thanks everyone for the help !
Organize Datasets documentation into four documentation types to improve clarity and discoverability of content.

**Content to add in the very short term (feel free to add anything I'm missing):**
- A discussion on why Datasets uses Arrow that includes some context and background about why we use Arrow. Would also be great to talk about Datasets speed and performance here, and if you can share any benchmarking/tests you did, that would be awesome! Finally, a discussion about how memory-mapping frees the user from RAM constraints would be very helpful.
- Explain why you would want to disable or override verifications when loading a dataset.
- If possible, include a code sample of when the number of elements in the field of an output dictionary aren’t the same as the other fields in the output dictionary (taken from the [note](https://huggingface.co/docs/datasets/processing.html#augmenting-the-dataset) here).