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/incubator/youtubedl-material/Chart.yaml

@@ -10,7 +10,7 @@ dependencies:
   version: 1.0.26
 description: YoutubeDL-Material is a Material Design frontend for youtube-dl.
 home: https://github.com/truecharts/apps/tree/master/charts/stable/youtubedl-material
-icon: https://truecharts.org/_static/img/appicons/youtubedl-material.png
+icon: https://truecharts.org/img/appicons/youtubedl-material.png
 keywords:
 - youtube
 kubeVersion: '>=1.16.0-0'