Add unpoly for dynamic content loading

[reakaleek]

Changes

• Adds unpoly to dynamically load content in "progressive enhancement"-way.

Notes

• I also evaluated htmx at Add htmx to dynamically load content #545
• 👍 However, unpoly feels more natural and straight forward to use and fits our use case well
• 👍 In the future we can also conditionally load content (e.g. the pages navigation)
• 👎 Unpoly creates a global variable window.up and it has no typescript support.
• 👎 When unpoly is initialized it messes up existing smooth scroll links. (e.g in ToC nav)

Result

cursorful-unpoly-1740046902275.mp4

[reakaleek]

This data is actually not needed anymore because we don't need to recreate the navigation state. But kept it here just in case.

[reakaleek]

Closing in favour of #545

Reason:

Unpoly somehow messes up the scroll smooth behaviour of the table of contents.