setup.py: add documentation link

this will make the doc link appear in the PyPI homepage

if this is OK, after the diff is accepted I'll mass commit the result of running the following script with mr run, which will add the Documentation links to all our packages:

#!/bin/bash

if ! [ -f setup.py ] ; then
    # nothing to do, skip
    exit 0
fi
if grep -q Documentation setup.py ; then
    # already done, skip
    exit 0
fi

repo_name=$(basename $(pwd))
doc_link="\"Documentation\": \"https://docs.softwareheritage.org/devel/${repo_name}/\","

sed -i "/\"Source\":/a\ \ \ \ \ \ \ \ ${doc_link}" setup.py

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Migrated from D3095 (view on Phabricator)