Document missing toolkit classes

[texnixe]

e.g. collection class

https://forum.getkirby.com/t/where-are-the-docs-for-collection-filter/13570/2

[distantnative]

Would be pretty easy to just add the Toolkit package to https://github.com/getkirby/getkirby.com/blob/master/content/1_docs/3_reference/%40/packages.txt

Then the basic ones would be automatically created. Question is if we want to add manual docs to some as well? And do we want to link to some specifically?