fix: change npm install and npm run dev to yarn according to docs

[skirianov]

Description

Previously after installation with the CLI, users were receiving the following message

To finish the installation, run the following commands:

cd vsf-magento-playground
npm install

Then, go to the https://docs.vuestorefront.io/magento/ to learn how to configure the project.
Once finished, you can start the project using this command:

npm run dev

Related Issue

Closing #6879

Motivation and Context

Better DX

How Has This Been Tested?

Built-in test suites:

Test Suites: 16 passed, 16 total Tests: 29 passed, 29 total

sergiikirianov@Sergiis-MBP-2 cli % ./bin/run generate store
? What's your project name? test
? Choose an integration template: Magento 2
████████████████████████████████████████ || 100%
Sucessfully generated your store at "./test"!
To finish the installation, run the following commands:

cd test
yarn

Then, go to the https://docs.vuestorefront.io/magento/ to learn how to configure the project.
Once finished, you can start the project using this command:

yarn dev

Screenshots:

Before:

After:

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have read the CONTRIBUTING document.

Changelog

• [ ] I have updated the Changelog (V1) v2 and mentioned all breaking changes in the public API.
• [ ] I have documented all new public APIs and made changes to existing docs mentioning the parts I've changed so they're up to date.

Tests

• [ ] I have written test cases for my code
• [x] I have tested my Pull Request on production build and (to my knowledge) it works without any issues
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

  I tested manually my code, and it works well with both:

• [ ] Default Theme
• [ ] Capybara Theme

Code standards

• [x] My code follows the code style of this project.
• [ ] I have followed naming conventions

Docs

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.