docs: document Azure OpenAI env vars in .env.template (#6840)

The server reads AZURE_API_KEY, AZURE_URL and AZURE_API_VERSION
(app/config/server.ts) for the Azure provider, but none of them were
listed in .env.template. Add them next to the OpenAI settings.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: TIANQIAN1238

.env.template

@@ -37,6 +37,15 @@ BASE_URL=
 # Default: Empty
 OPENAI_ORG_ID=
 
+# Azure OpenAI api key. (optional)
+AZURE_API_KEY=
+
+# Azure OpenAI endpoint url, e.g. https://{resource}.openai.azure.com. (optional)
+AZURE_URL=
+
+# Azure OpenAI api version, e.g. 2023-08-01-preview. (optional)
+AZURE_API_VERSION=
+
 # (optional)
 # Default: Empty
 # If you do not want users to use GPT-4, set this value to 1.