Host Engine Builds for Each PR via Vercel for Easier Testing

[mvaligursky]

It's fantastic that our engine PRs already build and deploy example projects to Vercel — this makes it super easy to share and review example changes.

However, it would be even more powerful if each PR also built and hosted the engine itself, using Vercel (or a similar service). This would allow contributors and reviewers to test changes directly in the Editor or any project without needing to manually build the engine locally.

Add the domain pattern engine-*****-playcanvas.vercel.app to the Editor’s list of allowed origins so it can load the PR-specific engine version seamlessly.

[mvaligursky]

additionally, build and hosting docs would be nice too :)