docs: add monospace font to API.md

[ca-d]

also to our css if missing

[stee-re]

@ca-d do you mean a common shared set of well defined/known css variable names for fonts?
I know we talked about having default fonts bundled at the oscd-shell level. I'm not 100% sure what we want at the API level.

[ca-d]

Sorry I didn’t notice yet: This is where this issue belongs, now that this is where API.md lives.