Add text classification tutorial

[weijinglok]

This PR introduces the text classification tutorial for the docs site.

This tutorial has been reviewed by @jwmueller offline.

[codecov]

Codecov Report

Merging #154 (d6fbff0) into master (3f17be6) will not change coverage.
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  Hits          901      901           
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[jwmueller]

We should standardize the tutorial filenames as well. I vote for the following names, can you update them if you agree?

image_quickstart_tutorial.ipynb
text_quickstart_tutorial.ipynb
tabular_quickstart_tutorial.ipynb

[jwmueller]

already edited offline

[anishathalye]

Following up from #163: I think the path notebooks/{modality}_quickstart_tutorial is unnecessarily long. How about something like quickstart/{modality} or tutorials/{modality}?

[weijinglok]

Following up from #163: I think the path notebooks/{modality}_quickstart_tutorial is unnecessarily long. How about something like quickstart/{modality} or tutorials/{modality}?

Done!

[jwmueller]

Following up from #163: I think the path notebooks/{modality}_quickstart_tutorial is unnecessarily long. How about something like quickstart/{modality} or tutorials/{modality}?

This will not work long-term IMO. Eg. There can be multiple tutorials about image, such as: image_quickstart, image_indepth, etc.

@weijinglok For this PR, your rename to tutorials/image.ipynb is fine with me. But we'll need to revisit it in the future when more tutorials are added.

[jwmueller]

LGTM!