add docs for filter.expr and wrapped payload stuff

src/content/docs/concepts/payload.md

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+---
+title: Payload
+sidebar:
+    order: 5
+---
+
+[Processors](/concepts/Processors) operator on a "wrapped" version of the message produces by the module. This wrapped version is available to all processors and is defined as follows:
+- **Payload**: the actual message produced by the module (or the result of the previous [processor](/concepts/processors) in the [route](/concepts/routes)).
+- **Sender**: the network address of the module that produced the message (if applicable not all modules will have a sender address).
+- **Modules**: a map of all [modules](/concepts/modules) in the router with the module id as the key and the module instance as the value.

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

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+---
+title: Expr Filter
+sidebar:
+    order: 2
+---
+The `filter.expr` processor evaluates an [Expr expression](https://expr-lang.org/playground). If the expression evaluates to true then the payload is sent through. This processor will return an error if the expression fails to evaluate.
+
+- **type**: `filter.expr`
+- **params**:
+    - **expression**: [Expr expression](https://expr-lang.org/playground) to evaluate. The expression has access to the [wrapped payload](/concepts/payload).
+
+### Example
+Match a payload great than or equal to 0
+```
+- type: filter.expr
+  params:
+    pattern: "Payload >= 0"
+```