Add copy button to code blocks

[Shrikantgiri25]

Description

Currently, code blocks in the DRF documentation must be copied manually. This PR introduces a “Copy” button for code blocks to improve usability.

Scope: Docs-only improvement — does not modify DRF library or API.

Implementation:

Files and Purpose:

1. css/copy-button.css – Styles the copy button, positions it inside code blocks, and applies DRF’s red theme color for consistency.

2. js/copy-button.js – Adds a “Copy” button to each <pre><code> block, handles the clipboard copy functionality, and provides user feedback (“Copied!”).
 
3. mkdocs.yml – Registers the CSS and JS files so they are loaded with the documentation.
 
4.  main.html – Ensures the extra CSS is included in the page layout.

Discussion - #9829

Screenshots: