autodoc: fix crash when a signature handler returns a signature for a data object

[aryansk]

Fixes #14576.

What

Documenting a callable data object (e.g. sig_bug = SigBug() with a #: docstring) crashed autodoc when an autodoc-process-signature event handler returned a replacement signature:

WARNING: error while formatting signature for module.sig_bug: list assignment index out of range [autodoc]

Data objects intentionally never get a signature, so signatures is empty when the autodoc-process-signature event fires. The handler's result was assigned to signatures[0], raising IndexError and dropping the object's docstring from the output.

Fix

When the event handler returns a replacement signature and signatures is empty, append it instead of assigning to index 0. Existing behavior (replacing the first signature) is unchanged.

Test

Added test_data_object_signature_from_event_handler, which registers a listener returning ('()', None) for data objects and documents target.callable.function end-to-end. It fails on main with the exact warning above and passes with this change.

Also added a CHANGES.rst entry under the in-development release.

[jdillard]

Closing as duplicate of #14578

[aryansk]

Thanks for the triage @jdillard — closing as a duplicate of #14578 makes sense; that fix covers the same crash. No further action needed from me here (and #14605 remains open if it's still useful).