Infra: errors blocking migration towards docutils 0.22+

Following the work in #4087, I have decided to investigate possible issues with going further, towards docutils 0.22+. The following are extracts from the errors failing the build process (tested on Sphinx 9.1.0 and docutils 0.22.4). I have error logs with full tracebacks and more details available if needed.

Errors

• peps/pep-0554.rst:29: (ERROR/3) Indirect hyperlink target (id="concurrency") refers to target "concurrency", which is a duplicate, and cannot be used as a unique reference.
• peps/pep-0683.rst:476: (ERROR/3) Indirect hyperlink target (id="documentation") refers to target "documentation", which is a duplicate, and cannot be used as a unique reference.
• peps/pep-0426.rst:748: (ERROR/3) Duplicate target name, cannot be used as a unique reference: "extras".
• peps/pep-0653.rst:722: (WARNING/2) Footnote content expected.
• peps/pep-0827.rst:1963: (WARNING/2) Footnote content expected.
• peps/pep-0697.rst:250: (ERROR/3) Indirect hyperlink target (id="reference-implementation") refers to target "reference implementation", which is a duplicate, and cannot be used as a unique reference.
• peps/pep-0791.rst:163: (ERROR/3) Indirect hyperlink target (id="id1") refers to target "motivation", which is a duplicate, and cannot be used as a unique reference.

All of those errors are coming from docutils, not from Sphinx, but they are hard-failing the parallel Sphinx builds.

CC @hugovk

EDIT: After investigation, only docutils is blocked. Sphinx 9 works fine.

Start by reproducing the parallel Sphinx build with Sphinx 9.1.0 and docutils 0.22.4, then inspect the reported locations in peps/pep-0554.rst, pep-0683.rst, pep-0426.rst, pep-0653.rst, pep-0827.rst, pep-0697.rst, and pep-0791.rst. Done means the listed docutils errors and warnings no longer hard-fail the build while preserving the PEP content.