Please use docutils instead of pypandoc

When reviewing the size of swh.web dependencies, @douardda noticed that we use pypandoc, which seems to pull a ton of extra dependencies.

After looking at the code, pypandoc is only used to render restructuredtext readme files;

We should consider switching over to docutils (which is already in use for the apidoc generation) or to a more lightweight module such as https://github.com/pypa/readme_renderer, rather than keep this extra dependency.

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Migrated from T2250 (view on Phabricator)