Add documentation on usage of Camel simple expression in configuring property

### Description

The documentation lack some explanation on how to use a [Camel Simple Expression](https://camel.apache.org/components/4.0.x/languages/simple-language.html) in a configuration property in the `application.properties` file.

Adding a section in the [user guide](https://github.com/apache/camel-quarkus/blob/main/docs/modules/ROOT/pages/user-guide/configuration.adoc) to using a config property for a Camel simple expression and explaining that expressions like `${body}` need special treatment (E.g using the raw value) would be very helpful.

As an example:
* this is a valid expression embedded as a parameter in the route: `expression: "${body} contains 'not_in'"`
* this is the same valid expression as a property: `myexpression=\\${body} contains 'not_in'`

Note: this is a follow up of this [discussion on zulip](https://camel.zulipchat.com/#narrow/stream/257302-camel-quarkus/topic/.E2.9C.94.20Kamelet.20simple-filter-action.20configuration).