Allow running demo_notebook in Google Colab

[pquentin]

Adding an "Open in Colab" button was easy, but making the actual notebook runnable in Colab took way more effort. Ive followed the conventions used in https://github.com/elastic/elasticsearch-labs but:

• I've used a hack to allow loading the test data from that notebook.
• Making this nicer (including using the cloud id instead of host/port) will require changes in the tests.

I was not sure who to ask for review, feel free to change the reviewers list.

[pquentin]

Deleted Jenkins webhooks manually again, closing/reopening.

[pquentin]

@joshdevins What are your thoughts on this new approach? I've tested that nox -s docs works when ES is active, and that the notebook can be fully run in Colab too. Fetching the data from GitHub does seem like a nice approach, we're talking about less than 2MB here.

[pquentin]

I've also enabled the Read the Docs PR builds, so you can see:

• the new API: https://eland--604.org.readthedocs.build/en/604/reference/io.html
• the full notebook: https://eland--604.org.readthedocs.build/en/604/examples/demo_notebook.html

[pquentin]

Not going to merge this as is as I realized that eland.etl exposes various functions doing the same thing as mine.

[pquentin]

Closing in favor of #630.