Remove HTML only for table examples documentation, raise Doxygen requirement

[vermeeren]

This issue is to be handled after Debian bullseye becomes stable.

#642 implemented several improvements to table rendering, which causes LaTeX errors with Doxygen <= 1.8.18. As a result, current documentation contains .. only:: html for table examples in documentation/source/tables.rst to avoid triggering the issue.

Debian bullseye contains Doxygen >= 1.9.1, which doesn't contain the issue. A while after bullseye becomes the next stable we can officially bump Doxygen requirement (also in README) and drop the .. only:: html from docs.