📝 Add documentation about settings and env vars (#1118)

* Add doc and example for env var config

* Syntax highlight for .env file

* Add test for configuration docs

* 📝 Update settings docs, add more examples

* ✅ Add tests for settings

* 🚚 Rename "Application Configuration" to "Metadata and Docs URLs"

to disambiguate between that and settings

* 🔥 Remove replaced example file

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs_src/metadata/tutorial003.py

@@ -0,0 +1,8 @@
+from fastapi import FastAPI
+
+app = FastAPI(docs_url="/documentation", redoc_url=None)
+
+
+@app.get("/items/")
+async def read_items():
+    return [{"name": "Foo"}]