CI: allow preview deployment of web pull request

[fariss]

To allow quick feedback from others, it would be really helpful to have a preview deployment of a pull request so others do not need to build it on their local environment.

An option would be Vercel GitHub Actions: it creates and deploys an instance upon a new PR (can limit to web PRs only), it also allows to inspect the build logs for failed builds (example). Vercel's preview URLs look like: <project-name>-git-<branch-name>-<scope-slug>.vercel.app Vercel can deploy separate sites like main, rules, explorer.

A flexible and lightweight alternative is Deploy to Vercel GitHub Action which allows for PR previews using Vercel

Another alternative that doesn't use Vercel is Deploy PR Preview GitHub Actions. The preview URL will be: https://[owner].github.io/[repo]/pr-preview/pr-[number]/ (⟵ could lead to defacement)