chore(apps): move icons into the apps folder (#3139)

* chore(apps): move icons into the apps folder

* rename to `icon.png`

* replace solr image

* remove `docs/` from gitignore

* move app specific docs into app folder [skip ci]

* fix some links

* chore(apps): update image links temporary

charts/stable/synapse/Chart.yaml

@@ -11,7 +11,7 @@ dependencies:
 deprecated: false
 description: A Helm chart to deploy a Matrix homeserver stack into Kubernetes
 home: https://github.com/truecharts/apps/tree/master/charts/stable/synapse
-icon: https://truecharts.org/_static/img/appicons/synapse.png
+icon: https://truecharts.org/img/appicons/synapse.png
 keywords:
 - chat
 - matrix