Always show Copy button in code blocks

[glin]

Is your feature request related to a problem? Please describe.

Follow-up from the PPM Quarto migration: https://positpbc.slack.com/archives/C087T1PQSDR/p1739993749177159?thread_ts=1739405653.405299&cid=C087T1PQSDR

Copy buttons in Quarto code blocks are hidden and only visible over mouse hover by default. There is a config option to make Copy buttons always visible, and we think this would be useful to enable across the docs for usability and accessibility:

• Better discoverability of the Copy button for all users
• Better discoverability of the Copy button for keyboard-only users
• Somewhat mitigates a bug with Copy buttons not appearing on keyboard focus (Code block copy button has no visible indicator on keyboard focus quarto-dev/quarto-cli#12118)
• Keeps consistency with the previous MkDocs Material site always showing Copy buttons
• https://quarto.org/ has also enabled Copy buttons always showing by default

Describe the solution you'd like

I'm not sure where this would be added, but the config option is at: https://quarto.org/docs/output-formats/html-code.html#copy-button

format:
  html:
    code-copy: true

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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