Add copy button to readthedocs

[pmocz]

Small quality-of-life enhancement: I love it when readthedocs has the copy button enabled for code blocks so I can easily grab the steps I need

[The9Cat]

Great idea, thanks! I didn't know about that extension, but it was easy enough to find and enable. Very nice.

[The9Cat]

Addressed in EXP-code/EXP-docs#31

View it here: https://exp-docs.readthedocs.io/en/copybutton/

[michael-petersen]

Just a short update: this has been ingested into the main branch of EXP-docs, so you should see this in action on all readthedocs pages!

Part of openjournals/joss-reviews#7302.

[michael-petersen]

Finished with the merge of #100.