Add SQL guide
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5223). All of your documentation changes will be reflected on that endpoint. I think we may want more content on this page that's not SQL related. Some of that content probably already lives in the main `load` docs page, but might be bad to remove major things like csv/pandas from there...WDYT we should do @lhoestq ? Maybe the main load page can only show one example and redirect to this page for more details ?

We can do the same for pandas stuff: have one example in load, and redirect to this page for more details The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5223). All of your documentation changes will be reflected on that endpoint.
This PR adapts @nateraw's awesome SQL notebook as a guide for the docs!