docs: Fix build failing due to the new cvs loader module

It was missing the inclusion of the README.rst and the readme was inconsistent with some other loader modules.

This commit fixes those issues and should fix the build [1]

• [1] https://jenkins.softwareheritage.org/view/swh-draft/job/DLDCVS/job/tests/3/console

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Migrated from D5967 (view on Phabricator)