Add Google Lighthouse Score

[mgifford]

We have this table here:
https://docs.gova11y.io/rules/a11y

May want to bring over the Google Lighthouse Score to see how it meshes with the critical, serious, moderate & minor from axe:
https://docs.google.com/spreadsheets/d/1sFG9r6pv9WVcPggmBRFQo2Ccy7-0pmy0AhVkB-Om1WQ/edit#gid=1027079035

Not sure that comparison has been made elsewhere.