Make notebooks easily executable

[bjnath]

We agreed at the 8 June 20 doc team meeting that interactivity is a vital feature of notebooks.

To ensure this, each tutorial will include:

1. A button to run the notebook in Binder
2. Also a button to run the notebook in Colab. Although we'd prefer that users choose Binder, Colab runs more quickly. We'd give users a choice.
3. A button to easily download the notebook for local execution.

Per @rossbar all three of these can be included.

Noting this here as a requirement for new notebooks.

[melissawm]

With the new format for the repository, I think this is more or less solved. The only remaining question is whether we should have a colab link. Personally, I think Binder fits our project better.

[rgommers]

Personally, I think Binder fits our project better.

I agree, I'd prefer to support the community-developed solution here. We don't need GPUs, so Binder fits the need - the slightly faster startup for me is not a good reason to add Colab.

Related: the second tutorial in this repo has "Colab" in the name in the README - that looks weird.