doc: Turn user_retirement_pipeline README.rst into actual reST
doc: Turn user_retirement_pipeline README.rst into actual reST

The README.rst file in the the user_retirement_pipeline role,
purportedly reStructuredText, was actually Markdown (with reST-ish
headings). This discrepancy led to the README being poorly rendered
when viewed online on GitHub.

Fix it to be actual reST, piping the original through pandoc.

stvstnfrd/configuration,mitodl/configuration,arbrandes/edx-configuration,EDUlib/configuration,EDUlib/configuration,edx/configuration,edx/configuration,EDUlib/configuration,stvstnfrd/configuration,mitodl/configuration,arbrandes/edx-configuration,EDUlib/configuration,stvstnfrd/configuration,stvstnfrd/configuration,arbrandes/edx-configuration,arbrandes/edx-configuration,edx/configuration,arbrandes/edx-configuration,mitodl/configuration,mitodl/configuration,edx/configuration,EDUlib/configuration,stvstnfrd/configuration