added pypi links in index.rst

`csst` is the pipeline package for the **CSST** (Chinese Space Station Telescope).

* github: https://github.com/csster/csst

* PYPI: https://pypi.org/project/csst

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Installation

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Install using `pip <https://pypi.org/project/matplotlib>`__:

   .. code-block:: bash

       pip install -U git+git://github.com/csster/csst

or

   .. code-block:: bash

       pip install -U csst

.. toctree::