Add copy button to docs

[pavithraes]

It'll be nice to be able to copy code snippets directly from the docs, sphinx-copybutton is a lightweight tool that can be used for this.

(Personally, I'd really appreciate this. I help support the Dask community, which involves copying code from the Zarr docs quite often.)

[joshmoore]

Good idea!

[tasansal]

made a pr #1124 @joshmoore

[joshmoore]

Thanks, @tasansal!