feat(docs): add copy button to code blocks

[yashksaini-coder]

Summary

• Adds a clipboard icon button to every <pre> code block in the docs, always visible (no hover to reveal).
• Clicking copies the code to clipboard via navigator.clipboard.writeText() and briefly shows a green checkmark for 2 s.
• Button is vertically centered on the right edge of the block — works correctly for both single-line and multi-line blocks.
• SVG icons are injected safely via DOMParser + appendChild — no innerHTML usage.
• Script loads only on doc pages via the scripts template block in layouts/doc/single.html — no impact on landing or other page types.

No new dependencies. No changes to markdown content or build config.

Fixes #343

Files changed

File	Change
website/assets/js/copy-code.js	New: injects icon copy button into each .docs-content pre via DOM APIs
website/assets/css/main.css	New .copy-btn styles: always visible, vertically centered, green on copied
website/layouts/doc/single.html	Adds scripts block to load the JS only on doc pages

Test plan

• Run hugo server in website/, open any doc page (e.g. /guides/sessions/)
• Confirm clipboard icon button is visible by default on all code blocks (no hover needed)
• Click it — icon switches to a green checkmark, clipboard contains the code text
• Verify button is vertically centered on single-line and multi-line blocks
• Verify no copy button appears on the homepage or Why Tau page
• Verify existing code block styling (dark background, syntax highlighting) is unchanged

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