feat(docs): add pinned copy-to-clipboard button for code blocks

[shekhar-narayan-mishra]

Summary

This PR adds a copy-to-clipboard button to all

 blocks in the Deno documentation.

Previously, users had to manually select and copy code snippets, which was inefficient and error-prone — especially for longer commands. This enhancement improves usability by adding a persistent, accessible copy button that works across all code blocks.
Features
✨ Adds a copy button pinned to the top-right corner of every code block
✨ Handles horizontal scrolling: button stays fixed while code scrolls
✨ Strips shell prompts like $, >, and ❯ before copying
✅ Displays a ✔️ check icon briefly after successful copy
✅ Styled to blend with the existing docs look (no emojis, clean UI)
Implementation
copy.client.ts: injects the copy button and handles copy logic
styles.css: includes styles for button positioning, appearance, and animation
Code is scoped and non-invasive; it de-duplicates any existing theme-based buttons
How to Test Locally
Run the dev server:
deno task serve
Visit any page with code examples (e.g. Install or Getting Started)
Hover over a code block — confirm:
Copy icon is pinned to top-right
It stays fixed during horizontal scroll
Clicking it copies the code (without $, >, etc.)
A check icon appears briefly then reverts
Linked Issue:
Closes #2745
Screenshots
Before:

After:

Notes for Reviewers
This is a non-breaking, client-side UX improvement
Does not modify build, runtime, or API logic
Happy to refine styling or placement if requested