Add way to exclude PRs from PR preview

[atteggiani]

Description

It would be good to have a way to exclude specific PRs from triggering PR previews if needed.

One possible way to achieve this, is by inserting a message in the pr body, something like

!NO-PREVIEW

Why

Sometimes, PRs might cause errors in the build/deployment of the website, or a preview might simply not be needed/wanted.

Close Criteria

Implement and document method to exclude PRs from the PR preview