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/healthchecks/Chart.yaml

@@ -8,7 +8,7 @@ description: Healthchecks is a cron job monitoring service. It listens for HTTP
   and email messages ("pings") from your cron jobs and scheduled tasks ("checks").
   When a ping does not arrive on time, Healthchecks sends out alerts.
 home: https://github.com/truecharts/apps/tree/master/charts/stable/healthchecks
-icon: https://truecharts.org/_static/img/appicons/healthchecks.png
+icon: https://truecharts.org/img/appicons/healthchecks.png
 keywords:
 - cron
 - monitoring