Make the copy field button always visible for better discoverability

## Description
User feedback on the [number_of_shards option page](https://www.elastic.co/docs/reference/elasticsearch/curator/option_number_of_shards)
indicated the copy field button only appears when hovering over code snippets. This can make the feature hard to find, especially for users unfamiliar with our docs UI.

## Proposed change
Update the docs UI so that the copy button is always visible, not just on hover. This will improve discoverability and accessibility for all users.