[docs] Update templates for starting the tutorial and an Expo Router app

[keith-kurak]

Why

The default template now has Expo Router, but the tutorial assumes just an App.js file.

How

Updated the command to use the blank template. Long-term, it's surely better to update the entire tutorial to use the default template, but we really gotta go through and test that, and maybe adjust some things to account for the default template having multiple tabs we don't need, etc.

Also added a callout explaining why we're not using the default template. Maybe it's too much info, but I feel like a lot of people with big ideas trying to learn RN are going to think "what if I want a second screen?" so why not answer their question right away.

That lead me to also update the Router starter, as that refers to the SDK 50 tabs template.

Test Plan

Look at it

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[keith-kurak]

@amandeepmittal I tried to make Expo Router more prominent at the end of the tutorial. I also tried adding further context about templates in create-a-project. I've had several conversions on Discord now where folks are confused about whether or not they have to create a new app with Expo Router, so I think it would help to somehow make it more clear in most cases where we suggest creating a new project that you can create a new project with or without Expo Router.

[keith-kurak]

also removed the Router compatibility table ... it seems redundant if you use npx expo install. Seems like maybe a vestige of when router wasn't in the monorepo.

[amandeepmittal]

Long-term, it's surely better to update the entire tutorial to use the default template, but we really gotta go through and test that, and maybe adjust some things to account for the default template having multiple tabs we don't need, etc.

If we decide to update project structure of the Tutorial example app, sure we can. But we also need to consider (at the current stage) that using Expo Router library isn't supported with Snack and this tutorial right now, to provide a more beginner friendly experience relies heavily on Snack.