Fix homepage and environment setup docs styling

[jonsamp]

Why

Addresses #4124.

After releasing the home page, I got a few points of feedback to address. This PR:

• Adds dark mode colors to the "Continue with Expo" button on /docs/environment-setup. Pictured below is the hovered state:
• Removes large format devices when viewing the site on a more narrow page:
  • Wide page shows all of the devices in the illustration
  • Narrow page shows only Android and iOS devices
• Added a new illustration for the Framework > Tools card. Previously it looked like Expo was offering tools that are provided by React Native. I updated it to show a pseudo-UI of Expo Orbit. I tried to make the image timeless (no mentions of the Pixel 8 or iPhone 15) since those will be dated quickly.
• Added a more recent video to the Watch section. Now it shows the following video, which is from React Conf 2024: https://www.youtube.com/live/0ckOUBiuxVY?si=PmlrPytEh9V_gDJw&t=2315

Test plan

Look at the home page and make sure that sections mentioned above look correct. Also click on "Get started" to go to /docs/environment-setup and make sure that the "Continue with Expo" button has the correct colors in light and dark mode.

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