Upgrade Sphinx docs toolchain (6.2.1 → 8.1.3)

[ChihweiLHBird]

Upgrade Sphinx docs toolchain (6.2.1 → 8.1.3)

What

Modernize the docs dependency group and sync the lockfile:

Package	Before	After
sphinx	6.2.1	8.1.3
sphinx_rtd_theme	2.0.0	3.1.0
docutils (transitive)	0.19	0.21.2
jinja2	(unpinned)	>=3.1.6
pygments	>=2.20.0,<3.0.0	unchanged

Why

• sphinx==6.2.1 cannot run on Python 3.12+ (ast.Str, removed in 3.12), blocking local doc builds on modern interpreters.
• 8.1.3 is the newest Sphinx that still supports the project's minimum Python (3.10), so requires-python is unchanged.
• sphinx_rtd_theme==3.1.0 supports Sphinx 6–9 and constrains docutils<0.23.

Changes

• pyproject.toml — bump the docs group as above.
• uv.lock — regenerated to match (RTD/CI use uv sync --frozen, which fails on a stale lock).
• .readthedocs.yml — removed the obsolete note about pinning an older Sphinx to fix bullet-point rendering. That was a sphinx_rtd_theme < 1.0 / docutils >= 0.17 issue, long fixed in the current theme.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.39%. Comparing base (2ec566d) to head (83c6948).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2165   +/-   ##
=======================================
  Coverage   83.39%   83.39%           
=======================================
  Files         351      351           
  Lines       81476    81476           
  Branches     8746     8746           
=======================================
  Hits        67940    67940           
  Misses      10744    10744           
  Partials     2792     2792           

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[ChihweiLHBird]

Wait, there's a v8.2.3, let me try that version Never mind, v8.2 doesn't support Python 3.10