Update docs for v 1.0.0 and Scala 3

[RobertSandiford]

Hi,

Just tried to get started, using Scala 3 JS, and found this page: https://japgolly.github.io/scalacss/book/quickstart/index.html

This didn't work with some cryptic errors about conflicting cross-version suffixes.

I figure it's because the quick start uses 0.8.0-RC1, and 0.8.0-RC1 does not support Scala 3. I tried with version 1.0.0 and it seems OK.

Page needs updating?

[RobertSandiford]

On the topic, the quickstart is not working for me (the 12 px syntax). Should this be working?