docs: Prepare for new sub tree

See #11925

docs/content/en/functions/collections/Union.md

@@ -1,46 +0,0 @@
----
-title: collections.Union
-description: Given two arrays or slices, returns a new array that contains the elements that belong to either or both arrays/slices.
-categories: []
-keywords: []
-action:
-  aliases: [union]
-  related:
-    - functions/collections/Complement
-    - functions/collections/Intersect
-    - functions/collections/SymDiff
-    - functions/collections/Union
-  returnType: any
-  signatures: [collections.Union SET1 SET2]
-aliases: [/functions/union] 
----
-
-Given two arrays (or slices) A and B, this function will return a new array that contains the elements or objects that belong to either A or to B or to both.
-
-```go-html-template
-{{ union (slice 1 2 3) (slice 3 4 5) }}
-<!-- returns [1 2 3 4 5] -->
-
-{{ union (slice 1 2 3) nil }}
-<!-- returns [1 2 3] -->
-
-{{ union nil (slice 1 2 3) }}
-<!-- returns [1 2 3] -->
-
-{{ union nil nil }}
-<!-- returns an error because both arrays/slices have to be of the same type -->
-```
-
-## OR filter in where query
-
-This is also very useful to use as `OR` filters when combined with where:
-
-```go-html-template
-{{ $pages := where .Site.RegularPages "Type" "not in" (slice "page" "about") }}
-{{ $pages = $pages | union (where .Site.RegularPages "Params.pinned" true) }}
-{{ $pages = $pages | intersect (where .Site.RegularPages "Params.images" "!=" nil) }}
-```
-
-The above fetches regular pages not of `page` or `about` type unless they are pinned. And finally, we exclude all pages with no `images` set in Page parameters.
-
-See [intersect](/functions/collections/intersect) for `AND`.