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/dependency/mongodb/Chart.yaml

@@ -7,7 +7,7 @@ dependencies:
 deprecated: false
 description: Fast, reliable, scalable, and easy to use open-source no-sql database system.
 home: https://github.com/truecharts/apps/tree/master/charts/dependency/mongodb
-icon: https://truecharts.org/_static/img/appicons/mongodb.png
+icon: https://truecharts.org/img/appicons/mongodb.png
 keywords:
   - mongodb
   - database