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In the table model used by docutils, the `cols` attribute of `tgroup`
nodes is mandatory, see [1]. It is used in texinfo builder in [2].

[1]: https://www.oasis-open.org/specs/tm9901.htm#AEN348
[2]: https://github.com/sphinx-doc/sphinx/blob/v2.1.2/sphinx/writers/texinfo.py#L1129

* Doc: Add texinfo support to the Makefile