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/uptimerobot-prometheus/Chart.yaml

@@ -6,7 +6,7 @@ dependencies:
   version: 10.2.1
 deprecated: false
 description: Prometheus Exporter for the official uptimerobot CLI
-icon: https://truecharts.org/_static/img/appicons/uptimerobot-prometheus.png
+icon: https://truecharts.org/img/appicons/uptimerobot-prometheus.png
 home: https://github.com/truecharts/apps/tree/master/charts/stable/uptimerobot-prometheus
 keywords:
 - uptimerobot