docs: add mypy plugin

docs/mypy_plugin.rst

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+[Experimental] Mypy plugin
+===========================
+
+Mypy plugin provides type checking for gokart tasks using Mypy.
+This feature is experimental.
+
+How to use
+--------------
+
+Configure Mypy to use this plugin by adding the following to your ``mypy.ini`` file:
+
+.. code:: ini
+
+    [mypy]
+    plugins = gokart.mypy:plugin
+
+or by adding the following to your ``pyproject.toml`` file:
+
+.. code:: toml
+
+    [tool.mypy]
+    plugins = ["gokart.mypy:plugin"]
+
+Then, run Mypy as usual.
+
+For example the following code linted by Mypy:
+
+.. code:: python
+
+    import gokart
+    import luigi
+
+
+    class Foo(gokart.TaskOnKart):
+        # NOTE: must all the parameters be annotated
+        foo: int = luigi.IntParameter(default=1)
+        bar: str = luigi.IntParameter(default=2)
+
+
+
+    Foo(foo=1, bar='2')   # OK
+    Foo(foo='1') # NG because foo is not int and bar is missing