Add markdown copy feature

[brookslybrand]

Why

Being able to copy a doc as markdown directly is a very useful feature when working with LLMs, chatbots, and AI-coding tools like Cursor.

Inspiration

Thanks Cloudflare and Vercel for the inspiration 🙏

What needs to be done

Thanks @kdcokenny for getting this started in #186 🤘

We want to take this opportunity to make some small tweaks to the docs pages on reactrouter.com

• Create dropdown for copying/editing pages
• Lift "edit" link into this dropdown
• Re-use the same dropdown functionality that's used for the branch and theme selectors in the main nav
  • Note: this means that the feature should be open-able before JS has loaded (progressive enhancement)
  • Bonus points if the "copy page" is a link before JS has loaded, since it won't be functional without JS
• Note: for the copy icon just use the existing icon in this repo. No need to switch to the svg in the design

Here are the various hover/focus states the dropdown can be in:

And here is where we want to position the dropdown on the page (note: for mobile this should stack under the "mode legend"):