✨ Add docs and tests for Jinja2 templates (#186)

* ✨ Add docs and tests for Jinja2 templates

* 🎨 Fix format in test, remove unused import

docs/tutorial/static-files.md

@@ -0,0 +1,34 @@
+You can serve static files automatically from a directory using <a href="https://www.starlette.io/staticfiles/" target="_blank">Starlette's Static Files</a>.
+
+## Install `aiofiles`
+
+First you need to install `aiofiles`:
+
+```bash
+pip install aiofiles
+```
+
+## Use `StaticFiles`
+
+* Import `StaticFiles` from Starlette.
+* "Mount" it the same way you would <a href="https://fastapi.tiangolo.com/tutorial/sub-applications-proxy/" target="_blank">mount a Sub-Application</a>.
+
+```Python hl_lines="2 6"
+{!./src/static_files/tutorial001.py!}
+```
+
+Then you could have a directory `./static/` with some files that will be served directly.
+
+## Details
+
+The first `"/static"` refers to the sub-path this "sub-application" will be "mounted" on. So, any path that starts with `"/static"` will be handled by it.
+
+The `directory="static"` refers to the name of the directory that contains your static files.
+
+The `name="static"` gives it a name that can be used internally by **FastAPI**.
+
+All these parameters can be different than "`static`", adjust them with the needs and specific details of your own application.
+
+## More info
+
+For more details and options check <a href="https://www.starlette.io/staticfiles/" target="_blank">Starlette's docs about Static Files</a>.

docs/tutorial/templates.md

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+You can use any template engine you want with **FastAPI**.
+
+A common election is Jinja2, the same one used by Flask and other tools.
+
+Starlette has utilities to configure it easily that you can use directly in your **FastAPI** application.
+
+## Install dependencies
+
+Install `jinja2`:
+
+```bash
+pip install jinja2
+```
+
+If you need to also serve static files (as in this example), install `aiofiles`:
+
+```bash
+pip install aiofiles
+```
+
+## Using `Jinja2Templates`
+
+* Import `Jinja2Templates` form Starlette.
+* Create a `templates` object that you can re-use later.
+* Declare a `Request` parameter in the *path operation* that will return a template.
+* Use the `templates` you created to render and return a `TemplateResponse`, passing the `request` as one of the key-value pairs in the Jinja2 "context".
+
+```Python hl_lines="4 11 15 16"
+{!./src/templates/tutorial001.py!}
+```
+
+!!! note
+    Notice that you have to pass the `request` as part of the key-value pairs in the context for Jinja2. So, you also have to declare it in your *path operation*.
+
+## Writing templates
+
+Then you can write a template at `templates/item.html` with:
+
+```jinja hl_lines="7"
+{!./src/templates/templates/item.html!}
+```
+
+It will show the `id` taken from the "context" `dict` you passed:
+
+```Python
+{"request": request, "id": id}
+```
+
+## Templates and static files
+
+And you can also use `url_for()` inside of the template, and use it, for example, with the `StaticFiles` you mounted.
+
+```jinja hl_lines="4"
+{!./src/templates/templates/item.html!}
+```
+
+In this example, it would link to a CSS file at `static/styles.css` with:
+
+```CSS hl_lines="4"
+{!./src/templates/static/styles.css!}
+```
+
+And because you are using `StaticFiles`, that CSS file would be served automatically by your **FastAPI** application at the URL `/static/styles.css`.
+
+## More details
+
+For more details, including how to test templates, check <a href="https://www.starlette.io/templates/" target="_blank">Starlette's docs on templates</a>.