[Feature] Provide first steps recommendation on the first time running with berry

Describe the user story

I have just started looking at berry-ing a project, I have questions like:

• should I delete my node_modules folder (I'm like 90% on yes)
• should I delete my yarn.lock (probably but with less confidence)
• there are now 4 new root files and folders ( .yarn, .yarnrc, .yarnrc.yml, .pnp.js) a one liner about why each exist would be 👍

Describe the solution you'd like

When berry sees: yarn.lock & node_modules and is creating yarnrc.yml for the first time. It can pretty safely assume a first time migration and finish the initial yarn install with some a congrats like:

Thanks for trying out yarn 2 🎉

Your next steps:

- Remove your node_modules with `rm -rf */node_modules` (or whatever it takes to recursively delete)
- Remove your yarn.lock from the repo
- We've added the following new files:
 - .yarn - keeps your modules in a zip file, you want to keep this git
 - pnp.js - helps tools understand where modules are
 - `yarnrc.yml` - the config for yarn v2
 - `.yarnrc` - the config for yarn v1 to tell it to use v2

You can find out more in https://next.yarnpkg.com/advanced/migration

Describe the drawbacks of your solution

It's a bit verbose, and you could argue that the .yarnrc was set from yarn policies set-version v2 which isn't this command

Describe alternatives you've considered

More docs in the migration guide?

Start with the migration guide linked in the issue and trace where the first Yarn Berry install detects an existing yarn.lock and node_modules while creating yarnrc.yml. Define the first-run message and its explanation of the new files, cleanup steps, and migration link; done means the recommendation appears only for that migration case and is covered by an appropriate test.