Insert Feed Mixer Solution-Guide (#211)

Co-authored-by: Christoph Apke <c.apke@dke-data.com>

Author: capke

modules/ROOT/nav.adoc

@@ -85,4 +85,5 @@
 ** xref:ids-and-definitions.adoc[IDs and Definitions]
 ** xref:keys.adoc[agrirouter Keys]
 ** xref:limitations.adoc[Limitations]
-** xref:urls.adoc[URLs]
+** xref:urls.adoc[URLs]
+** xref:feedmixer-solution-guide.adoc[Feed Mixer Solution-Guide]