Allow domain nesting in TOC

[minijackson]

Is your feature request related to a problem? Please describe.

I'm developing a Nix domain for Sphinx, and before Sphinx 7.4.0 I've been able to use domain-defined nesting, which changed in #12367. Documenting NixOS options is kinda like documenting a JSON schema, where the keys can be heavily nested, so nesting them in document doesn't look good.

Here's an example of what I did with Sphinx 7.3.7 that I can't reproduce in 7.4.0+: https://minijackson.github.io/sphinxcontrib-nixdomain/examples/rst-autodoc-example.html#using-automodule

Describe the solution you'd like
A partial revert of #12367, where it would be somehow possible to choose between document nesting or domain nesting in the TOC.

Describe alternatives you've considered

Put the full name of each object in the TOC, but I find it way less readable.

Additional context

Before #12367:

After #12367 and showing the full name in the TOC: