add documentation for filter.regex

src/content/docs/processors/string/filter.md

@@ -1,18 +0,0 @@
----
-title: String Filter
-sidebar:
-    order: 4
----
-The `string.filter` processor takes a string and turns it into an array of bytes. This processor will return an error if the message being processed is not a string.
-
-- **type**: `string.filter`
-- **params**:
-    - **pattern**: regex pattern to match against the incoming string
-
-### Example
-Match strings starting with `hello`
-```
-- type: string.filter
-  params:
-    pattern: "^hello.*"
-```