add string.filter processor

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

@@ -0,0 +1,18 @@
+---
+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.*"
+```