docs: Better document dataclasses extension

Author: pawamoy

docs/extensions/built-in/dataclasses.md

@@ -2,4 +2,52 @@
 
 The `dataclasses` extension adds support for [dataclasses][] from the standard library. It works both statically and dynamically. When used statically, it re-creates the `__init__` methods and their signatures (as Griffe objects), that would otherwise be created at runtime. When used dynamically, it does nothing since `__init__` methods are created by the library and can be inspected normally.
 
-**This extension is enabled by default.**
+Example:
+
+```python
+from dataclasses import dataclass
+
+
+@dataclass
+class Room:
+    uid: int
+    name: str
+    capacity: int = 10
+    available: bool = True
+```
+
+With the `dataclasses` extension enabled, the Griffe object for the `Room` class will get an `__init__` method with the following signature:
+
+```python
+def __init__(self, uid: int, name: str, capacity: int = 10, available: bool = True) -> None:
+    ...
+```
+
+Additional metadata like `ClassVar`, the `init` and `kw_only` parameters, or the `KW_ONLY` sentinel are also recognized and will update the `__init__` method signature accordingly.
+
+**This extension is enabled by default.** It is always added last. If you need to give it a higher priority, you can explictly enable it to change its position in the list of extensions (it will run only once):
+
+=== "CLI"
+    ```console
+    $ griffe dump -e dataclasses,other my_package
+    ```
+
+=== "Python"
+    ```python
+    import griffe
+
+    my_package = griffe.load("my_package", extensions=griffe.load_extensions("dataclasses", "other"))
+    ```
+
+=== "mkdocstrings"
+    ```yaml title="mkdocs.yml"
+    plugins:
+    - mkdocstrings:
+        handlers:
+          python:
+            options:
+              extensions:
+              - dataclasses
+              - other
+    ```
+