fix: Expand relative cross-references against the object the docstring was written on - #342

With inherited_members enabled, the docstring of a base class member is rendered in the docs of the inheriting class. In AutorefsHook.expand_identifier, leading-dot relative cross-references found in such docstrings walk up from current_object, which for an inherited member is a Griffe alias living under the inheriting class. A reference written on pkga.sub.Base.MAPPING therefore expands to pkgb.Thing in the docs of a package that subclasses Base, instead of pkga.Thing, silently linking to a wrong target or aborting strict builds when the consumer has relative_crossrefs disabled.

This change anchors the dot-walk on the docstring's parent, the object the docstring was actually written on, when that object is reachable, and falls back to the current object otherwise. This mirrors how get_context already uses docstring.parent for source location. The reproducer and root-cause analysis in #341 describe exactly this behavior, including the frequenz-sdk build failure against frequenz-quantities.

A regression test in tests/test_rendering.py builds two in-memory modules with cross-module inheritance and asserts a ....Thing reference in an inherited member's docstring expands to pkga.Thing; it fails on main and passes with this change. A control assertion also checks that non-inherited objects still expand identically.

Re-export aliases are anchored uniformly by this change: since docstring.parent of a re-export alias points at the private defining object, relative references there now expand against it. If different treatment is preferred for re-exports, happy to follow maintainer guidance. Whether the consumer's relative_crossrefs setting should gate expansion of a dependency's docstring at all is out of scope for this PR.

Fixes #341