add workflow with website test

[ralfhandl]

One of the recent dependabot updates introduced 85 warnings when building the website, before we were at zero warnings.

If a future dependabot update introduces errors, it will break the site without warning.

• add test that verify successful build
• add tests for important site features, for example that the navigation sidebar contains key menu items, or that the /oas page lists all major spec versions (we broke both in the past without noticing it)