Published Documentation is Outdated

The published documentation for Python Manual Installation here: https://dynet.readthedocs.io/en/latest/python.html#manual-installation is outdated, as it includes installing Eigen from https://bitbucket.org/eigen/eigen instead of https://github.com/clab/dynet/releases/download/2.1/eigen-b2e267dc99d4.zip as shown here: https://github.com/clab/dynet/blob/master/doc/source/python.rst

so we need to publish that to reflect on https://dynet.readthedocs.io/en/latest/python.html#manual-installation

Compare the published Python Manual Installation section at dynet.readthedocs.io/en/latest/python.html#manual-installation with doc/source/python.rst. Start by checking how this repository publishes its documentation, then ensure the published page reflects the Eigen release URL shown in the source; done when the online section is updated.

Written by the indexing model from the issue text.