Host tutorial on doc website (spdx.github.io)

[bact]

To address #30 (comment), host the tutorial directly on https://spdx.github.io/spdx-python-model/tutorial/

We use nbconvert to convert the notebook to HTML.

Once published, the tutorial will be at
https://spdx.github.io/spdx-python-model/tutorial/using-spdx3.html

(it is a copy from https://gist.github.com/bact/7227ad858500c2097a25344a4af015d6 )

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Any Python notebooks placed in tutorial/ dir in the repo will be published.

[bact]

See demo

• tutorial https://bact.github.io/spdx-python-model/tutorial/using-spdx3.html
• link from homepage https://bact.github.io/spdx-python-model/