feat: scoped registries

[nvandessel]

Changes

• Add documentation relevant to the use of scopedRegistryUrl and registryScopes
• Add documentation for private scoped registries and using the UPM_REGISTRY_TOKEN environment variable

Checklist

• Read the contribution guide and accept the
  code of conduct
• Readme (updated or not needed)
• Tests (added, updated or not needed)

[github-actions]

[GabLeRoux]

should be merged only when related PR is merged 👍 Nice work.

[nvandessel]

Thank you so much! Appreciate the keen eye finding that typo! 😬

[GabLeRoux]

Let's merge this. Only missing thing to do is to fix code formatting with yarn lint. 👍

[GabLeRoux]

I wasn't able to update your branch, but I was able to create a new branch in this repository here: https://github.com/game-ci/documentation/tree/feature/scoped-registries

[nvandessel]

@GabLeRoux I'm fine with closing this one and opening another from the other branch if that's easiest!

I also just went and ran the yarn lint from gh codespaces, so if it's preferable to merge this PR seeing as it's linked in the others then that works too. Whatever works and is best for you!