MAINT: Refresh the docs toolchain: unpin Sphinx, drop vendored sphinxext, deduplicate floors

[mmcky]

Problem

• docs/rtd-requirements.txt pins sphinx<=6.2.1 (April 2023; Sphinx is on 8.x) while the RTD image is otherwise current — an increasingly tight constraint on theme/extension versions and a latent docs-build failure.
• The same file re-declares runtime floors (numba>=0.49, numpy>=1.17, scipy>=1.5) — a second copy of metadata the install-metadata issue in this milestone is changing; it will drift (and already has).
• docs/sphinxext/ vendors three ~2014 IPython/matplotlib extensions (ipython_directive.py, ipython_console_highlighting.py, only_directives.py). conf.py puts the directory on sys.path but its extensions list loads IPython.sphinxext.* — the maintained upstream versions — so the vendored copies are pure dead weight.

Proposed change

1. Delete docs/sphinxext/ and the sys.path.insert for it in conf.py; confirm the IPython-shipped directives cover current usage.
2. Unpin Sphinx (or float a modern floor, sphinx>=8); rebuild on RTD; fix whatever surfaces (typically theme/config deprecations).
3. Collapse the duplicated runtime floors: the RTD config already pip-installs the package, so rtd-requirements.txt should hold docs-only tools (sphinx, theme, numpydoc, ipython, matplotlib).

Content/generator work (#854, #852, #169) is out of scope — this is the toolchain under it.

Acceptance criteria

• Clean RTD build on current Sphinx; no docs/sphinxext/; zero new build warnings (or enumerated and issue-linked)
• Runtime dependency versions stated in exactly one file (pyproject.toml)

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From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).