Small documentation clean up items (#9144)

* docs clean up

- remove `important` call out - should all be replaced with `note` or `warning`
- remove unnecessary indent in code snippet for 3.0 release notes

* Update docs/releases/3.0.md

Co-authored-by: Dan Braghis <dan@zerolab.org>

docs/getting_started/tutorial.md

@@ -19,10 +19,10 @@ $ python3 --version
 
 If this does not return a version number or returns a version lower than 3.7, you will need to [install Python 3](https://www.python.org/downloads/).
 
-```{important}
-   Before installing Wagtail, it is necessary to install the **libjpeg** and **zlib** libraries, which provide support for working with JPEG, PNG and GIF images (via the Python **Pillow** library).
-   The way to do this varies by platform—see Pillow's
-   [platform-specific installation instructions](https://pillow.readthedocs.org/en/latest/installation.html#external-libraries).
+```{note}
+Before installing Wagtail, it is necessary to install the **libjpeg** and **zlib** libraries, which provide support for working with JPEG, PNG and GIF images (via the Python **Pillow** library).
+The way to do this varies by platform—see Pillow's
+[platform-specific installation instructions](https://pillow.readthedocs.org/en/latest/installation.html#external-libraries).
 ```
 
 ### Create and activate a virtual environment

docs/releases/3.0.md

@@ -244,13 +244,13 @@ The `form_data` field in the `AbstractFormSubmission` model (and its subclasses
 
 Example change
 
-```
-    def process_form_submission(self, form):
-        self.get_submission_class().objects.create(
-            # form_data=json.dumps(form.cleaned_data, cls=DjangoJSONEncoder),
-            form_data=form.cleaned_data, # new
-            page=self, user=form.user
-        )
+```python
+def process_form_submission(self, form):
+    self.get_submission_class().objects.create(
+        # form_data=json.dumps(form.cleaned_data, cls=DjangoJSONEncoder),
+        form_data=form.cleaned_data, # new
+        page=self, user=form.user
+    )
 ```
 
 ### Removed `size` argument from `wagtail.utils.sendfile_streaming_backend.was_modified_since`