Add copy-to-clipboard controls for install snippets and package citations

[Copilot]

Package and installation pages expose a few high-frequency copy targets, but currently require manual selection. This adds lightweight copy controls to the install commands and package citation block without broad template churn.

• Scope

  • adds copy buttons to package installation snippets
  • adds copy buttons to install page command blocks
  • adds a copy action for the package citation content after it is loaded dynamically

• Template changes

  • marks install-oriented <pre> blocks as opt-in copy targets via data-copyable
  • adds a citation copy button to the package detail header near the rendered citation content

• Client-side behavior

  • extends code_blocks.js to:
    • inject copy buttons for pre[data-copyable]
    • copy either the associated code block or an explicit target (data-copy-target)
    • show short-lived success / failure state on the button
    • fall back to document.execCommand("copy") when the Clipboard API is unavailable
  • re-runs copy-button initialization after package citations are inserted asynchronously

• Styling

  • adds compact copy-button styles for code blocks
  • reserves space at the top of copyable code blocks so the button does not overlap content
  • styles the citation copy control consistently with the existing package page UI

• Example

  <pre data-copyable>
  <code class="dark nohighlight">
  if (!require("BiocManager", quietly = TRUE))
    install.packages("BiocManager")
  BiocManager::install("ExamplePkg")
  </code>
  </pre>