docs: Prepare for new sub tree

See #11925

docs/content/en/functions/math/Counter.md

@@ -1,35 +0,0 @@
----
-title: math.Counter
-description: Increments and returns a global counter.
-categories: []
-keywords: []
-action:
-  aliases: []
-  related: []
-  returnType: uint64
-  signatures: [math.Counter]
----
-
-The counter is global for both monolingual and multilingual sites, and its initial value for each build is 1.
-
-```go-html-template
-{{ warnf "single.html called %d times" math.Counter }}
-```
-
-```sh
-WARN  single.html called 1 times
-WARN  single.html called 2 times
-WARN  single.html called 3 times
-```
-
-Use this function to:
-
-- Create unique warnings as shown above; the [`warnf`] function suppresses duplicate messages
-- Create unique target paths for the `resources.FromString` function where the target path is also the cache key
-
-[`warnf`]: /functions/fmt/warnf
-[`resources.FromString`]: /functions/resources/fromstring
-
-{{% note %}}
-Due to concurrency, the value returned in a given template for a given page will vary from one build to the next. You cannot use this function to assign a static id to each page.
-{{% /note %}}