📝 Add documentation for Behind a Proxy - Proxy Forwarded Headers, using --forwarded-allow-ips="*" (#14028)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs_src/behind_a_proxy/tutorial001_01.py

@@ -0,0 +1,8 @@
+from fastapi import FastAPI
+
+app = FastAPI()
+
+
+@app.get("/items/")
+def read_items():
+    return ["plumbus", "portal gun"]