Add copy button to documentation code blocks

[aclark4life]

No description provided.

[timgraham]

I think it could be confusing not to include shell prompts. By that logic, should we have no >>> in Python console sessions so they are more copyable? Anything with more than one command or an output after the command isn't copyable anyway, e.g.

$ python manage.py makemigrations admin auth contenttypes
Migrations for 'admin':
  mongo_migrations/admin/0001_initial.py
    - Create model LogEntry
...

For comparison, the console examples in Django's docs have a linux/windows toggle and no copy button (example: https://docs.djangoproject.com/en/dev/intro/tutorial01/). A copy button is only present on code snippets.