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/mattermost/Chart.yaml

@@ -10,7 +10,7 @@ dependencies:
   version: 8.0.24
 description: Mattermost is an open source platform for secure collaboration across the entire software development lifecycle.
 home: https://github.com/truecharts/apps/tree/master/charts/stable/mattermost
-icon: https://truecharts.org/_static/img/appicons/mattermost.png
+icon: https://truecharts.org/img/appicons/mattermost.png
 keywords:
 - team
 - mattermost