Add page on models and varinfos, part 1 (#663)

* Add page on models and varinfos

* fix deps

* Use new DPPL release

* Minor text fixes

* Update developers/models/varinfo-overview/index.qmd

Co-authored-by: Markus Hauru <mhauru@turing.ac.uk>

* Mention that `unflatten` + `evaluate!!` is still faster

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Co-authored-by: Markus Hauru <mhauru@turing.ac.uk>

Author: penelopeysm