docs/_thread: Add a placeholder docs for _thread module.

Doesn't list specific API calls yet, the purpose is to let user know that
the module exists.

docs/library/_thread.rst

+:mod:`_thread` -- multithreading support
+========================================
+
+.. module:: _thread
+   :synopsis: multithreading support
+
+|see_cpython_module| :mod:`python:_thread`.
+
+This module implements multithreading support.
+
+This module is highly experimental and its API is not yet fully settled
+and not yet described in this documentation.

docs/library/index.rst

@@ -88,6 +88,7 @@ it will fallback to loading the built-in ``ujson`` module.
        ustruct.rst
        utime.rst
        uzlib.rst
+       _thread.rst
 
 .. only:: port_pyboard
 
@@ -114,6 +115,7 @@ it will fallback to loading the built-in ``ujson`` module.
        ustruct.rst
        utime.rst
        uzlib.rst
+       _thread.rst
 
 .. only:: port_wipy