docs: add copy button to docs code blocks

[SHYXIN]

What changed

Adds a one-click Copy button to every code block in the docs content area (issue #343).

• website/assets/js/copy-code.js (new): on DOMContentLoaded, injects a <button class="copy-btn"> into each .docs-content pre. Clicking copies the code via navigator.clipboard.writeText() and shows a brief Copied! confirmation (or Failed if the clipboard write is rejected).
• website/assets/css/main.css: styles .copy-btn — absolutely positioned top-right, hidden until the parent <pre> is hovered or focused (keyboard accessible via :focus-visible), and flashes green on success. Docs code blocks get top padding so the button sits clear of the first line instead of overlapping it.
• website/layouts/doc/single.html: loads the script only on doc pages through the existing scripts block in baseof.html, following the same Hugo Pipes pattern already used for nav-toggle.js (resources.Get "js/copy-code.js" → js.Build | minify | fingerprint in production).

No new dependencies, and no impact on landing or other page types.

Related issue

Fixes #343

Test approach

• node --check website/assets/js/copy-code.js passes (JS syntax valid).
• Built locally with hugo --minify (Hugo extended 0.165.0): the site builds clean (35 pages), the copy-code asset is emitted (minified + fingerprinted), and it is loaded only on docs pages (the landing page does not reference it).
• Verified the button visually with headless Edge: it appears on hover/focus, copies on click (Copied!), and — after the top-padding fix — no longer overlaps the first code line (measured: button bottom ≈ 569px < code top ≈ 580px). Screenshots for all three states are attached in the PR comment.

Checklist

• Only files related to this task were changed
• No Python/test changes needed (docs-site change only)
• Follows the existing Hugo Pipes asset-loading convention
• Visual result verified locally (hugo --minify build + screenshots in comment); maintainer review welcome

[SHYXIN]

Verification (local hugo --minify build, Hugo extended 0.165.0)

• The Copy button is injected on docs pages only and is hidden until the parent <pre> is hovered or focused.
• Top padding added to docs code blocks so the button sits in the clear space above the code and no longer overlaps the first line.
• Clicking copies the code to the clipboard and the button flashes a "Copied!" confirmation.

Measured bounding boxes confirmed the button no longer overlaps the first code line (button bottom < code top).