feat: add copy-to-clipboard button on all code blocks

[aayushi-singhhh]

Adds a 'Copy' button to every Rouge-highlighted code block across the entire documentation and tutorial sections of the site.

Changes

js/copy-code.js (new file)

• Pure vanilla JS, zero dependencies, IIFE-scoped
• Targets all div.highlight > pre blocks (Rouge output) and bare pre > code blocks not wrapped by the highlighter
• Uses the modern async navigator.clipboard.writeText() API
• Falls back to a hidden textarea + document.execCommand('copy') for non-HTTPS pages and older browsers
• Button shows 'Copied!' (green) on success, 'Failed' (red) on error, auto-resets to 'Copy' after 2 seconds
• Button is disabled during the async copy to prevent double-clicks
• Guards against duplicate buttons if attachCopyButtons() is called twice

css/customstyles-precice.css (appended, ~65 lines)

• .copy-code-btn: absolutely positioned top-right of the code block, hidden (opacity:0) by default, fades in on div.highlight:hover
• Visible on :focus for keyboard navigation
• @media (hover: none): always visible on touch/mobile devices
• .copy-code-btn--success: green feedback (#d4edda / #28a745)
• .copy-code-btn--error: red feedback (#f8d7da / #dc3545)
• .copy-code-btn:disabled: default cursor while copy is in-flight

_includes/head.html (1 line added)

• Loads js/copy-code.js after customscripts.js on every page

Why this matters

preCICE documentation is heavily code-heavy (C++, Python, XML config, shell commands). Users currently must manually select and copy code, which is error-prone for long config blocks. This brings the site's DX in line with ReadTheDocs, GitHub, and MkDocs Material, all of which provide copy buttons as a baseline feature.

No dependencies added. No existing files modified beyond customstyles-precice.css (append-only) and head.html (1 line). Build output verified: jekyll build completes with zero errors/warnings.

[MakisH]

Thank you for your contribution!

This adds some new scripts, which would not be needed once we move to a better framework (#472), which would give a better integrated button very easily.

Since this would now be additional code to maintain, and since it still would need some theming and further review to get it right, I think we should not focus our energy into this direction right now.

In any case, it's a good idea, and I hope you learned something while making it.