Adds smoke tests

[ViMaSter]

Summary

Wanted to add automated version updates, but realized we don't have any way to make sure the website still works after we upgrade packages.
We currently do deploy it to preview.studio-lovelies.com for each PR, but it might just not respond or return a 404; GitHub wouldn't care and declare the deployment completed successfully.

This PR adds test that ensure that the home page / works and /404 actually returns a 404.
Consider this a smoke test for the web server.

Also updates the README to contain everything previously only established via chat.

Testing instructions

1. Make sure what the README says actually works
2. Check that preview.studio-lovelies.com still looks fine

Additional information

• [ ] Changes UI
• [ ] Has associated resource:

[ViMaSter]

Snuck in two performance improvements that cause the Largest Contentful Paint (LCP) to occur earlier:

• 61973cf: fonts are now pre-loaded
• 3d00bcd: resized the logo images to x2 of their largest rendered size (90px in height); the original files still exist tho

Page should feel a bit snappier now. 👀