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/gotify/Chart.yaml

@@ -10,7 +10,7 @@ keywords:
   - server
   - gotify
   - messages
-icon: https://truecharts.org/_static/img/appicons/gotify.png
+icon: https://truecharts.org/img/appicons/gotify.png
 sources:
   - https://gotify.net/
   - https://github.com/gotify/server