docs: add a FastAPI transition documentation

docs/guide/fastapi_transition.md

@@ -0,0 +1,55 @@
+## Returning data from API calls
+**old:**
+```python
+return (
+    {
+        "id": wallet.wallet.id, 
+        "name": wallet.wallet.name, 
+        "balance": wallet.wallet.balance_msat
+    },
+    HTTPStatus.OK,
+)
+```
+FastAPI returns `HTTPStatus.OK` by default id no Exception is raised
+
+**new:**
+```python
+return {
+    "id": wallet.wallet.id, 
+    "name": wallet.wallet.name, 
+    "balance": wallet.wallet.balance_msat
+}
+```
+
+To change the default HTTPStatus, add it to the path decorator
+```python
+@core_app.post("/api/v1/payments", status_code=HTTPStatus.CREATED)
+async def payments():
+    pass
+```
+
+## Raise exceptions
+**old:**
+```python
+return (
+    {"message": f"Failed to connect to {domain}."},
+    HTTPStatus.BAD_REQUEST,
+)
+```
+
+**new:**
+
+Raise an exception to return a status code other than the default status code.
+```python
+raise HTTPException(
+    status_code=HTTPStatus.BAD_REQUEST,
+    detail=f"Failed to connect to {domain}."
+)
+```
+## Possible optimizations
+### Use Redis as a cache server
+Instead of hitting the database over and over again, we can store a short lived object in [Redis](https://redis.io) for an arbitrary key.
+Example:
+* Get transactions for a wallet ID
+* User data for a user id
+* Wallet data for a Admin / Invoice key