[Docs] Missing links in the architecture page

[ikedaosushi]

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Nothing
• Modin version (modin.__version__): 0.7.4
• Python version: Nothing
• Code we can use to reproduce: Nothing

Describe the problem

In architecture.html, the links of Pandas on Ray and Pandas on Dask is missing.

Source code / logs

https://github.com/modin-project/modin/blame/master/docs/architecture.rst#L205-L206

.. _Pandas on Ray: UsingPandasonRay/optimizations.html
.. _Pandas on Dask: UsingPandasonDask/optimizations.html