Feat/add copy page button

[Anderson-Andre-P]

Closes #13062

cc @johnpryan

Summary

Adds a "Copy page" button to the options dropdown (⋮) in the header of documentation pages, making it easy to copy the full page content to paste into an AI agent prompt.

How it works

• Clicking "Copy page" fetches the raw Markdown from GitHub (raw.githubusercontent.com) using the page's existing sourceUrl and writes it to the clipboard.
• On success, the button label changes to "Copied!" with a checkmark icon, and a toast notification appears at the bottom of the screen. Both reset after 2 seconds.
• The button only appears on pages with more than 150 words of body content (front matter and HTML tags excluded), hiding it from navigation/index pages that have no meaningful content to copy.

Test plan

• Click "Copy page" on a content page and paste into a text editor — confirm raw Markdown is copied
• Confirm the button label changes to "Copied!" and the toast appears on success
• Confirm the button is absent on navigation/index pages (e.g. /ui, /cookbook, /tools)
• Confirm the button is present on content pages (e.g. /ui/layout/constraints, /cookbook/networking/fetch-data)

[Anderson-Andre-P]

@johnpryan