docs: improve docs looks and feel

[antfu]

• Make Caslon font global, also applying to the the UI
  • /cc @orta do you remember where you got the CaslonOS-Regular.otf font? Could we have a bold version as well?
• Install Monaospace Argon for Monospace font to match the style
• Add a bit of yellow-tint to the foreground color on darkmode

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[orta]

I used https://www.1001fonts.com/caslon-os-font.html which is OTF, but only in regular - maybe https://github.com/ertekinno/libre-caslon-condensed can replace?