Provide documentation on how to style adodown side

[arthur-shaw]

• Point to Quarto documentation
• Say where to put SCSS/CSS in adodown file structure

[kbjarkefur]

This relates to adodownr but I think this issue should still be on this repo.

I think all user-facing documentation should exist in this repo as I think the tool on https://github.com/lsms-worldbank/adodownr will in 99% of the cases run as a GHA job in the background. And then users might not even know that there is another repo to look for additional documentation in.