[auth] Add custom Doppler icon (#1935)

## Description

Adds a custom icon for [Doppler](https://www.doppler.com)

auth/assets/custom-icons/_data/custom-icons.json

@@ -86,6 +86,9 @@
     {
       "title": "Discourse"
     },
+    {
+      "title": "Doppler"
+    },
     {
       "title": "dus.net",
       "slug": "dusnet"

auth/assets/custom-icons/icons/doppler.svg

@@ -0,0 +1 @@
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