Better links for models in README and doc index
# What does this PR do?

This PR fixes the links to the docs for unreleased models and makes the automatic copy to the index.rst a little bit better (by using relative links so there is no jump in version).
It adds instructions in the setup.py to clean the master in the links for unreleased models at the time of a release, we will just need to remember to have the right links in the README in a PR that adds a new model.

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Fixes #7657