Allow domain objects to be excluded from index

[mvorisek]

discussion: https://github.com/markstory/sphinxcontrib-phpdomain/pull/48#issuecomment-1639953391

needed for https://github.com/atk4/ui/pull/2087, atk4/ui docs index is wanted to contain only regular headers

[n-peugnet]

If what you mean by "index" is the "table of contents", then there is indeed an option in Sphinx to disable it: https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-toc_object_entries