Add job for GH pages preview

[juliapampus]

Feature Request

Add an action that allows to preview the GH pages rendering for changes introduced by open PRs.

Which Areas Would Be Affected?

workflows

Type: non-normative

Why Is the Feature Desired?

Currently, one need to preview the respec version locally or within repositories/orgs containing a working branch to ensure that file changes won't break the GH pages rendering.

Solution Proposal

Add workflow that temporarily deploys the GH pages on opened/updated PRs.

[arnoweiss]

I'm struggling to think of a way to implement this. What if there's multiple open PRs? When/How should branch deployments be cleaned up? How do we ensure a PR doesn't break the deployment on main?

[mkollenstart]

Perhaps we could use something like https://github.com/marketplace/actions/deploy-pr-preview