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/vikunja/Chart.yaml

@@ -14,7 +14,7 @@ dependencies:
   version: 3.0.24
 description: The to-do app to organize your life.
 home: https://github.com/truecharts/apps/tree/master/charts/stable/vikunja
-icon: https://truecharts.org/_static/img/appicons/vikunja.png
+icon: https://truecharts.org/img/appicons/vikunja.png
 keywords:
 - vikunja
 kubeVersion: '>=1.16.0-0'