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/fireflyiii/docs/validation.md

@@ -0,0 +1,13 @@
+# Input Validation
+
+__`APP_KEY`__
+Accepted formats are:
+
+- Letters, Numbers, Symbols, Exactly 32 characters (eg. `!oqVA9o2@br#$6vAyk8LLrDm54X5EtjD`) - It accepts `a-z`, `A-Z`, `0-9` and `!@#$%^&*?`
+
+You can try live [here](https://regex101.com/r/OR879w/1)
+Regex used to match this: `[a-zA-Z0-9!@#$%^&*?]{32}`
+
+---
+
+_If you find a field that you think it needs validation, please open an issue on github_