Add "Copy as Markdown" button to each page

[barredterra]

Summary

Each documentation page should offer a Copy as Markdown button so readers can quickly copy the page content in Markdown format (e.g. for pasting into an LLM, notes, or another editor).

Acceptance criteria

• A "Copy as Markdown" control is available on every page
• Clicking it copies the page’s Markdown source to the clipboard
• Provide brief user feedback on success (e.g. toast / button state)