📝 Update docs, add first tutorials

docs/tutorial/first-steps.md

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+The simplest FastAPI file could look like this:
+
+```Python
+{!tutorial/src/first-steps/tutorial001.py!}
+```
+
+Copy that to a file `main.py`.
+
+Run the live server:
+
+```bash
+uvicorn main:app --debug
+```
+
+!!! note
+    The command `uvicorn main:app` refers to:
+
+    * `main`: the file `main.py` (the Python "module").
+    * `app`: the object created inside of `main.py` with the line `app = FastAPI()`.
+    * `--debug`: make the server restart after code changes. Only use for development.
+
+You will see an output like:
+
+```hl_lines="4"
+INFO: Started reloader process [17961]
+INFO: Started server process [17962]
+INFO: Waiting for application startup.
+INFO: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
+```
+
+That last line shows the URL where your app is being served, in your local machine.
+
+### Check it
+
+Open your browser at <a href="http://127.0.0.1:8000" target="_blank">http://127.0.0.1:8000</a>. 
+
+You will see the JSON response as:
+
+```JSON
+{"hello": "world"}
+```
+
+### Interactive API docs
+
+Now go to <a href="http://127.0.0.1:8000/docs" target="_blank">http://127.0.0.1:8000/docs</a>. 
+
+You will see the automatic interactive API documentation (provided by <a href="https://github.com/swagger-api/swagger-ui" target="_blank">Swagger UI</a>):
+
+![Swagger UI](https://fastapi.tiangolo.com/img/index/index-01-swagger-ui-simple.png)
+
+
+### Alternative API docs
+
+And now, go to <a href="http://127.0.0.1:8000/redoc" target="_blank">http://127.0.0.1:8000/redoc</a>.
+
+You will see the alternative automatic documentation (provided by <a href="https://github.com/Rebilly/ReDoc" target="_blank">ReDoc</a>):
+
+![ReDoc](https://fastapi.tiangolo.com/img/index/index-02-redoc-simple.png)
+
+## Recap, step by step
+
+### Step 1: import `FastAPI`
+
+```Python hl_lines="1"
+{!tutorial/src/first-steps/tutorial001.py!}
+```
+
+`FastAPI` is a Python class that provides all the functionality for your API.
+
+### Step 2: create a `FastAPI` "instance"
+
+```Python hl_lines="3"
+{!tutorial/src/first-steps/tutorial001.py!}
+```
+
+Here the `app` variable will be an "instance" of the class `FastAPI`.
+
+This will be the main point of interaction to create all your API endpoints.
+
+This `app` is the same one referred by `uvicorn` in thet command:
+
+```bash
+uvicorn main:app --debug
+```
+
+If you create your app like:
+
+```Python hl_lines="3"
+{!tutorial/src/first-steps/tutorial002.py!}
+```
+
+And put it in a file `main.py`, then you would call `uvicorn` like:
+
+```bash
+uvicorn main:my_awesome_api --debug
+```
+
+### Step 3: create an endpoint
+
+```Python hl_lines="6"
+{!tutorial/src/first-steps/tutorial001.py!}
+```
+
+The `@app.get("/")` tells **FastAPI** that the function right below is an endpoint and that it should go to the path route `/`.
+
+### Step 4: define the endpoint function
+
+```Python hl_lines="7"
+{!tutorial/src/first-steps/tutorial001.py!}
+```
+
+This is a Python function. 
+
+It will be called by FastAPI whenever it receives a request to the URL "`/`".
+
+In this case, it is an `async` function.
+
+---
+
+You could also define it as a normal function instead of `async def`:
+
+```Python hl_lines="7"
+{!tutorial/src/first-steps/tutorial003.py!}
+```
+
+To know the difference, read the section about [Concurrency and `async` / `await`](/async/).
+
+### Step 5: return the content
+
+```Python hl_lines="8"
+{!tutorial/src/first-steps/tutorial001.py!}
+```
+
+You can return a `dict`, `list`, singular values as `str`, `int`, etc.
+
+You can also return Pydantic models (you'll see more about that later).
+
+There are many other objects and models that will be automatically converted to JSON.