Docs: Body Font + Size

[tylermcrobert]

Describe the problem

It's currently really tough to read the docs with the current body typography (subjectively of course). I'm finding myself frustrated when trying to process the information. FWIW it seems that I'm not alone [1] [2]

Describe the proposed solution

A reconsideration of the current typography choice. I've got suggestions but I'll keep them to myself for now.

Importance

nice to have

[brunnerh]

Duplicate of:

• [omnisite docs] Contrast and font - terrible for my eyesight svelte.dev#517
• Font readability issue in documentation svelte.dev#552
• [DOCS] Small serif text is hard to read on digital displays svelte.dev#639

Also, there should be a font switcher in the top right by now.