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:

[p-r-a-v-i-n]

I may be completely wrong here, but adding new features or code formatting changes is not allowed according to the DRF policy. This would violate the DRF policy.
As per the policy, DRF is already feature complete. though the requested feature relates to the documentation, it involves code changes, so I don't think it will be accepted.

[Shrikantgiri25]

I may be completely wrong here, but adding new features or code formatting changes is not allowed according to the DRF policy. This would violate the DRF policy.
As per the policy, DRF is already feature complete. though the requested feature relates to the documentation, it involves code changes, so I don't think it will be accepted.

Thanks for the feedback.
This PR doesn’t introduce any DRF features or library changes.
It only updates the documentation theme (CSS/JS), and DRF’s contributing guide explicitly states that documentation improvements are welcome.
If the maintainers prefer to avoid UX changes in the docs, I’m happy to follow their direction.