doc8 emits false positives for link targets defined in rst_prolog

[Julian]

Given a conf.py containing:

project = "foo"
copyright = "2022, Julian Berman"
author = "Julian Berman"

rst_prolog = """
.. _example: https://example.com
"""

with an example link defined in the rst_prolog, and a simple index.rst using the target:

Foo
===

This is an `example`_.

doc8 emits:

~/Desktop/foo/docs is a git repository on main 
⊙  doc8 .                                                                                                                         julian@Airm ●
Scanning...
Validating...
./index.rst:4: D000 Unknown target name: "example".
/Users/julian/.dotfiles/.local/share/virtualenvs/doc8/lib/python3.10/site-packages/doc8/parser.py:69: DeprecationWarning: The frontend.OptionParser class will be replaced by a subclass of argparse.ArgumentParser in Docutils 0.21 or later.
  opt = frontend.OptionParser(components=[parser], defaults=defaults)
/opt/homebrew/Cellar/[email protected]/3.10.9/Frameworks/Python.framework/Versions/3.10/lib/python3.10/optparse.py:1000: DeprecationWarning: The frontend.Option class will be removed in Docutils 0.21 or later.
  option = self.option_class(*args, **kwargs)
========
Total files scanned = 1
Total files ignored = 0
Total accumulated errors = 1
Detailed error counts:
    - doc8.checks.CheckCarriageReturn = 0
    - doc8.checks.CheckIndentationNoTab = 0
    - doc8.checks.CheckMaxLineLength = 0
    - doc8.checks.CheckNewlineEndOfFile = 0
    - doc8.checks.CheckTrailingWhitespace = 0
    - doc8.checks.CheckValidity = 1

~/Desktop/foo/docs is a git repository on main 
⊙  doc8 --version                                                                                                                 julian@Airm ●
1.0.0

flagging the link as an error, but this is incorrect, and make html will succeed just fine (and produce a page linking to the correct link target).