Added doc for switching elasticsearch to infinity (#3370)

### What problem does this PR solve?

Added doc for switching elasticsearch to infinity

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

docker/.env

@@ -1,3 +1,14 @@
+# The type of doc engine to use.
+# Supported values are `elasticsearch`, `infinity`.
+DOC_ENGINE=${DOC_ENGINE:-elasticsearch}
+
+# ------------------------------
+# docker env var for specifying vector db type at startup
+# (based on the vector db type, the corresponding docker
+# compose profile will be used)
+# ------------------------------
+COMPOSE_PROFILES=${DOC_ENGINE}
+
 # The version of Elasticsearch.
 STACK_VERSION=8.11.3