Add SQL guide

[stevhliu]

This PR adapts @nateraw's awesome SQL notebook as a guide for the docs!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[nateraw]

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 ?

[mariosasko]

I agree with @nateraw and think we should expand the existing "Load" page with more SQL examples instead of starting a new page unless there is enough content related to tabular data to put there (it seems that's not the case for now).

PS: CSV should be the preferred way to store/load tabular data IMO, due to its ease of use.

[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

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[lhoestq]

Cool thanks !