Add GHA workflow ensuring the website is working

[lampajr]

Setup a new job that checks whether the Horreum website content is working as expected.

Requirements:

• Must be triggered on pull requests and on master push
• Must be triggered only when there is at least one change in docs/site directory
• Run the build with Hugo (see hyperfoil.github.io/.github/workflows/deploy-gh-pages.yaml for more details)

Stretch goals (optional)

• Host a temporary static website for the pull request, if triggered.

Additional context

• Hosting a temporary website might be tricky as it might not be possible on Github, so it would require external integration that might not be worth the effort. That's why the goal is considered as optional.