feat: rework text for specificity/hierarchy

[jessicaschilling]

In this PR

• Closes https://github.com/ipfs-shipyard/ipfs-share-files/issues/86 by increasing prominence of "leave this page open" messaging on sender page
• Closes https://github.com/ipfs-shipyard/ipfs-share-files/issues/85 by giving recipients a tailored landing page with better entry points for understanding what IPFS is
• Re-orders information hierarchy so headline is actually at the top of the page for mobile widths; adds subhead value statement in case user doesn't read all the way down
• Rewrites all remaining text for clarity, specificity, hierarchy
• Adds more specific "Learn more" links throughout BUT this would really benefit from customized language in modals instead, as noted in https://github.com/ipfs-shipyard/ipfs-share-files/issues/82

Screenshots

(Note that CI also generates a pinned-to-cluster view if you want to play with it yourself without building locally.)

Sender view, initial state, desktop width

Sender view, files added, desktop width

Sender view, initial state, narrow width

Sender view, files added, narrow width

Recipient view, desktop width

Recipient view, narrow width