improve webdev and pagespeed scores

[richterdavid]

Google's public website measurement tools indicate that knative.dev and the pages on that site could use some improvements.

https://web.dev/measure/
https://developers.google.com/speed/pagespeed/insights/?url=knative.dev&tab=mobile
https://developers.google.com/speed/pagespeed/insights/?url=knative.dev&tab=desktop
https://www.thinkwithgoogle.com/feature/testmysite/
testmysite report: https://storage.googleapis.com/gweb-mobile-hub-test-my-site.appspot.com/public/reports/efd38062ead9419386ce04fdcfc45461.pdf

I mainly looked at the homepage, but testmysite averages across the whole domain. I'm guessing we use some shared css etc across the whole site, and that fixing the homepage will improve most/all other pages as well, but others should be checked too.

Possibly of interest: https://developers.google.com/speed/

Sounds like the current best tool from Google is https://web.dev/vitals/

https://webmasters.googleblog.com/2020/05/evaluating-page-experience.html

Forked from knative/docs#2526