feat(docs): add copy as markdown button to documentation pages (#2214)

[LestlinRobins]

Description

This PR adds a "Copy as Markdown" button to TKET documentation pages to make documentation content easily reusable in personal notes, technical documents, and AI coding assistants.

Key Implementation Details:

• Clean Markdown Source Fetching: Fetches Sphinx's native source file (_sources/<page>.md.txt) instead of parsing the DOM, ensuring that raw Markdown formatting (math, code blocks, lists, headers) is preserved cleanly without copying navigation, headers, footers, or page chrome.
• UI & Theme Integration: Integrates the button into .content-icon-container alongside the existing page actions (such as "View this page"), with full light/dark theme adaptation.
• Responsive Layout: Displays both icon and text on desktop viewports while automatically collapsing to a compact icon-only button on mobile screens ($\le 67\text{em}$).
• Clipboard Handling & Feedback: Uses navigator.clipboard.writeText with visual feedback (Copied! / Failed to copy) and includes a fallback for older environments.
• Compatibility: Coexists seamlessly with the existing sphinx-copybutton extension for individual code blocks.

Related issues

Closes #2214

Checklist

• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.

[LestlinRobins]

@cqc-alec I’ve made the required changes to support this feature. Please review them whenever you get a chance.

[LestlinRobins]

@CalMacCQ All checks have passed successfully. When you have a chance, could you please review the PR and merge it if everything looks good? Thank you!

[LestlinRobins]

@CalMacCQ Hi, could you please review the PR and merge it if everything looks good?
Thank you so much!