chore: improve lighthouse score

[LBBO]

Improves the docs' lighthouse score through various measures.

[vinckr]

How do I test if this improves lighthouse score?

This PR Lighthouse score:

Current deploy score:

In my test the deploy from this PR was worse than the current one, so I am not sure if I did something wrong in the test or if there is another issue. I ran the lighthouse tests from an incognito tab.

[LBBO]

That's actually a good point. It's kind of hard to get a deterministic result there and there might well be some unintended issues. But what you should™️ be able to tell deterministically, is that two issues used to be there that are now gone (image height & width and caching policy, IIRC). You can also make things more uniform by testing against two preview builds (just use another branch's preview), as Vercel may be applying certain optimisations to prod that it doesn't apply to previews. I definitely didn't see such a drastic difference in my tests, but it's totally possible that I did something wrong as well.

[vinckr]

This was on another preview (https://docs-fzoojem2y-ory.vercel.app/docs/welcome), no idea why it is higher now 😅

Have to take a more detailed look tomorrow.

[aeneasr]

The previews are typically a bit worse than the live site due to cloudflare optimizations. But if they're worse than other previews then we need to check

[mszekiel]

@vinckr please remember to test with https://pagespeed.web.dev/ for consistent results