Demo: add print.css (and related things)

[rejhgadellaa]

Related Issue

Demo for resolving (some of) #40

Summary of Changes

[X] Add print.css with some basic styling

• Hide site header, navigation
• Default sans-serif font face
• Basic page-breaks (always before h2, avoid page-breaks for sections)

[X] Add print.js

• Expand/collapse TOC when print dialog is opened (note: doesn't work with Dev Tools > Rendering > Emulate @media print)
• Temp: wrap every h2 chapter in a <section /> to be able to apply styling like page-breaks, etc. If we keep this approach, we pbbly need to move this into an 11ty plugin instead of client-side js!

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