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/etherpad/Chart.yaml

@@ -6,7 +6,7 @@ name: etherpad
 description: A real-time collaborative editor scalable to thousands of simultaneous real time users.
 type: application
 home: https://github.com/truecharts/apps/tree/master/charts/stable/etherpad
-icon: https://truecharts.org/_static/img/appicons/etherpad.png
+icon: https://truecharts.org/img/appicons/etherpad.png
 keywords:
   - etherpad
   - editor