Link demo docs on the main docs page (#255)

I hope this improves SEO, as all our great demos are not scraped by Google at the moment

## Description

*Please add a description here. This will become the commit message of the merge request later.*

Author: sbernauer

docs/modules/ROOT/nav.adoc

@@ -7,16 +7,7 @@
 ** xref:commands/services.adoc[]
 ** xref:commands/stack.adoc[]
 * xref:demos/index.adoc[]
-** xref:demos/airflow-scheduled-job.adoc[]
-** xref:demos/data-lakehouse-iceberg-trino-spark.adoc[]
-** xref:demos/hbase-hdfs-load-cycling-data.adoc[]
-** xref:demos/jupyterhub-pyspark-hdfs-anomaly-detection-taxi-data.adoc[]
-** xref:demos/logging.adoc[]
-** xref:demos/nifi-kafka-druid-earthquake-data.adoc[]
-** xref:demos/nifi-kafka-druid-water-level-data.adoc[]
-** xref:demos/spark-k8s-anomaly-detection-taxi-data.adoc[]
-** xref:demos/trino-iceberg.adoc[]
-** xref:demos/trino-taxi-data.adoc[]
+include::partial$demos.adoc[]
 * xref:customization/index.adoc[]
 ** xref:customization/add_demo.adoc[]
 ** xref:customization/add_stack.adoc[]

docs/modules/ROOT/pages/demos/index.adoc

@@ -4,6 +4,10 @@ The pages below this section guide you on how to use the demos provided by Stack
 
 To install a demo please follow the xref:quickstart.adoc[quickstart guide] or have a look at the xref:commands/demo.adoc[demo command].
 
+We currently offer the following list of demos:
+
+include::partial$demos.adoc[]
+
 [IMPORTANT]
 .External Components in these demos
 ====

docs/modules/ROOT/pages/index.adoc

@@ -47,3 +47,8 @@ Demos are installed with the xref:commands/demo.adoc[] command.
 A demo needs a stack to run on.
 To achieve this a demo has a dependency on a stack which gets automatically installed when a demo is installed.
 The stack in turn will install the needed Stackable release.
+
+You can browse the available demos xref:demos/index.adoc[on the demo page].
+We currently offer the following list of demos:
+
+include::partial$demos.adoc[]

docs/modules/ROOT/partials/demos.adoc

@@ -0,0 +1,10 @@
+**** xref:demos/airflow-scheduled-job.adoc[]
+**** xref:demos/data-lakehouse-iceberg-trino-spark.adoc[]
+**** xref:demos/hbase-hdfs-load-cycling-data.adoc[]
+**** xref:demos/jupyterhub-pyspark-hdfs-anomaly-detection-taxi-data.adoc[]
+**** xref:demos/logging.adoc[]
+**** xref:demos/nifi-kafka-druid-earthquake-data.adoc[]
+**** xref:demos/nifi-kafka-druid-water-level-data.adoc[]
+**** xref:demos/spark-k8s-anomaly-detection-taxi-data.adoc[]
+**** xref:demos/trino-iceberg.adoc[]
+**** xref:demos/trino-taxi-data.adoc[]