Add a "Copy as markdown" button on every doc page

[Zodiarche]

User Story

As a reader of the docs
I want a button on each page that copies the page's markdown source to my clipboard
So that I can quickly share the content (Discord, notes app), quote it elsewhere, or paste it into an LLM for follow-up questions

Issue description

Several modern doc sites (Anthropic, Vercel, Astro, Cloudflare, MDN…) ship a "Copy markdown" affordance on each page. It removes the friction of selecting text, formatting it back to markdown, and copying — particularly useful when answering questions on Discord with a verbatim snippet of the doc, or when feeding a page to an AI assistant.

Tasks

• Add a button on every doc page that copies the source markdown to the clipboard
• Decide placement (top-right of the article, near the title, or in the breadcrumb area)
• Show a brief confirmation toast after copy
• Source the markdown from the page's imported file (Docusaurus exposes it via useDoc() or similar)
• Cover both EN and FR

Notes

• Could also expose markdown via a ?format=markdown query or a .md URL suffix (some sites do that and it's LLM-friendly)
• Possibility of multiple variants: "Copy as markdown", "Open in ChatGPT", "Open in Claude"