Feature: add copy button for code blocks using sphinx-copybutton

[psobolewskiPhD]

Description

This PR adds the nice copy button to code blocks via sphinx-copybutton
https://github.com/executablebooks/sphinx-copybutton
E.g.

I've found these kinds of buttons common in other python docs (e.g. scikit-image)
You can see it at work in a PR to my fork: psobolewskiPhD#5
Here's the built docs artifact: https://github.com/psobolewskiPhD/napari-docs/actions/runs/3725012413

Note that the extension allows for customization such as stripping out prompts, etc. but for now this is just the basics.
https://sphinx-copybutton.readthedocs.io/en/latest/index.html

Type of change

• Fixes or improves existing content
• Adds new feature
• Fixes or improves workflow, documentation build or deployment

References

https://sphinx-copybutton.readthedocs.io/en/latest/index.html

Final checklist:

• My PR is the minimum possible work for the desired functionality
• I have commented my code, particularly in hard-to-understand areas
• I have added alt text to new images included in this PR

[brisvag]

Love it!

[alisterburt]

❤️