Add font to docs

[rivercory]

Added Poppins font for better readability!

[github-actions]

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[chris48s]

I think the choice docusaurus/infima have made (defaulting to system-ui) is really good as it makes it look "native" on a range of platforms. I don't see a good reason to diverge from that.
https://github.com/facebookincubator/infima/blob/450b41af48b2d4b670a483228b59360553885e3b/packages/core/styles/common/variables.pcss#L115-L117
I'll leave this open for a bit in case another maintainer wants to strongly disagree with me.

(If we were to change this, we should not use fonts.googleapis.com. We should pull in from https://www.npmjs.com/package/@fontsource/poppins and provide some more thoughtful fallbacks but lets not get bogged down in that for now)

[PyvesB]

I think the choice docusaurus/infima have made (defaulting to system-ui) is really good as it makes it look "native" on a range of platforms. I don't see a good reason to diverge from that.

Agreed!