[pkg] Add missing entries to MANIFEST.in
Detected by running `check-manifest
<https://github.com/mgedmin/check-manifest>`_. Add a tox environment
accordingly.
Closes #15229018.
[theme]
inherit = pyramid
pygments_style = sphinx.pygments_styles.PyramidStyle
stylesheet = cubicweb.css
[options]
logo = logo-cubicweb.svg
favicon = cubicweb.ico
in_progress = false
outdated = false
canonical_url =