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/wikijs/Chart.yaml

@@ -8,7 +8,7 @@ keywords:
 - wiki
 - wikijs
 home: https://github.com/truecharts/apps/tree/master/charts/stable/wikijs
-icon: https://truecharts.org/_static/img/appicons/wikijs.png
+icon: https://truecharts.org/img/appicons/wikijs.png
 sources:
 - https://hub.docker.com/r/linuxserver/wikijs/
 - https://github.com/Requarks/wiki