* Fix: Parent URL should be undefined if parent is falsy
* Document `docs.11tydata.js` better
* Move `docs/installation.md` to `docs/`, fixes#585
* Just in case
- Sidebar, overview listings, breadcrumbs now based on actual parent-child relationships, rather than increasingly outdated heuristics
- parent properties are now generated automatically from the URL structure, and need only be specified to override that default
- Ability to group by page hierarchy in overview pages, where pages that have >= 2 children become categories
Smaller improvements:
- More flexible syntax for specifying the params of overview pages
- [Overviews] Hide group heading if only one group is present
- parentItem and parentUrl properties that can be used on any page
- Alias a collection as the children of a page (useful for "virtual" parents like Layout)
- Do not error if a page card icon is missing
