Implement separate tests for website

[Maspital]

Right now, we don't have any way to check if GitHub Pages can correctly build our website other than:

• pushing to main and seeing whether or not if fails
• manually changing the build target for our website from main to <other branch>, checking if it works, then changing it back

Neither solution is elegant, and as for example can be seen in #26, there may be situations where we unknowingly break our website. Most promising solution is probably to configure a runner to test this stuff locally: https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/testing-your-github-pages-site-locally-with-jekyll

[Maspital]

Closing this for now, as the structure of GitHub actions we have right now is, in my opinion, pretty transparent, especially given that we start these workflows manually.