Docs: Move Dark Mode Toggle from Footer to Header for Better UX

### What is the documentation issue? The dark/light mode toggle on the Next.js documentation site is currently placed in the footer. While it works well functionally, its position makes it a bit difficult to find—especially for new visitors exploring the docs for the first time. Suggestion: It might improve discoverability and overall usability if the theme toggle were moved to the header area. This is a more common layout in modern documentation sites (such as [React](https://react.dev/), [Vue](https://vuejs.org/), and [MDN](https://developer.mozilla.org/zh-TW/)) and helps users easily notice and switch between themes without needing to scroll to the bottom of the page. ### Is there any context that might help us understand? Here’s a screenshot of the current footer where the dark/light mode toggle appears: When viewing longer documentation pages (for example, [Installation](https://nextjs.org/docs/app/getting-started/installation)), users need to scroll all the way down to find the toggle. For newcomers visiting the documentation, this can be confusing — they may want to switch to dark mode immediately but can’t easily locate the control. Placing the toggle in the header would make it immediately visible and provide a more intuitive, accessible experience for all users. <img width="1866" height="897" alt="Image" src="https://github.com/user-attachments/assets/4e274cca-5628-4d74-ae50-dc3fae001df0" /> ### Does the docs page already exist? Please link to it. _No response_