add distributions doc

doc/source/index.rst

@@ -27,6 +27,7 @@
    jittor.mpi
    jittor.linalg
    jittor.console
+   jittor.distributions
 
 
 .. toctree::

doc/source/jittor.distributions.md

@@ -0,0 +1,10 @@
+jittor.distributions
+=====================
+
+这里是Jittor的随机分布模块的API文档，您可以通过`from jittor import distributions`来获取该模块。
+
+```eval_rst
+.. automodule:: jittor.distributions
+   :members:
+   :undoc-members:
+```