add FAQ about licensing server error

docs/3.0/gem-server-authentication.md

@@ -141,3 +141,13 @@ docker build --build-arg BUNDLE_PACKAGER__DEV=$BUNDLE_PACKAGER__DEV
 docker compose build --build-arg BUNDLE_PACKAGER__DEV=xxx
 ```
 :::
+
+## FAQ
+
+Frequently asked questions:
+
+:::option `Forbidden 403`
+If you're seeing this error `Retrying download gem from https://packager.dev/avo-hq/ due to error (1/4): Gem::RemoteFetcher::FetchError bad response Forbidden 403`, this probably means that bundler does not have access to the `BUNDLE_PACKAGER__DEV` environment variable.
+
+Please read the guides above on how to set that on your development machine and in deployment scenarios.
+:::