[Docs] Update `EnumDocumenter` for newer Sphinx (#8914)

[Docs] Update `EnumDocumenter` for newer Sphinx (#8914)

The `no_docstring` argument of
`sphinx.ext.autodoc.Documenter.add_content()` has been
[deprecated](https://www.sphinx-doc.org/en/master/changes.html) since
Sphinx 3.4.0 (released in December 2020). It seems that newer Sphinx
versions have removed that argument, which breaks our `EnumDocumenter`
(e.g., see the following
[example](https://github.com/cvc5/cvc5/runs/7081097179?check_suite_focus=true)).
This commit updates our use of
`sphinx.ext.autodoc.Documenter.add_content()`.