docs: add docs for mypy plugin

doc/mypy.rst

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+Mypy plugin
+===========================
+
+Mypy plugin provides type checking for ``luigi.Task`` using Mypy.
+
+Require Python 3.8 or later.
+
+How to use
+--------------
+
+Configure Mypy to use this plugin by adding the following to your ``mypy.ini`` file:
+
+.. code:: ini
+
+    [mypy]
+    plugins = luigi.mypy
+
+or by adding the following to your ``pyproject.toml`` file:
+
+.. code:: toml
+
+    [tool.mypy]
+    plugins = ["luigi.mypy"]
+
+Then, run Mypy as usual.
+
+Examples
+--------
+
+For example the following code linted by Mypy:
+
+.. code:: python
+
+    import luigi
+
+
+    class MyTask(luigi.Task):
+        foo: int = luigi.IntParameter()
+        bar: str = luigi.Parameter()
+
+    MyTask(foo=1, bar='bar')
+
+    Foo(foo=1, bar='2')   # OK
+    Foo(foo='1', bar='2')  # Error: Argument 1 to "Foo" has incompatible type "str"; expected "int"