docs: Migrate to gp-sphinx workspace packages

[tony]

Summary

Migrates tmuxp docs from individually-managed Sphinx extensions and manual conf.py boilerplate to the gp-sphinx shared documentation platform.

This PR depends on git-pull/gp-sphinx#init-2 — the workspace restructuring branch that splits gp-sphinx into 4 separate packages. Not ready to merge until gp-sphinx publishes those packages to PyPI.

What changed

• pyproject.toml — Replace 9 individual sphinx deps (furo, sphinx-autodoc-typehints, sphinx-inline-tabs, sphinxext-opengraph, sphinx-copybutton, sphinxext-rediraffe, sphinx-design, myst-parser, linkify-it-py) with gp-sphinx + sphinx-argparse-neo. Uses [tool.uv.sources] pointing to local editable gp-sphinx packages during development.
• docs/conf.py — Rewrites 293-line manual config to a 46-line merge_sphinx_config() call; make_linkcode_resolve() replaces 50-line manual linkcode_resolve(); theme switches from furo to sphinx-gptheme (Furo child theme); spa-nav.js/tabs.js workarounds handled by gp-sphinx.
• docs/_ext/ — Removes sphinx_fonts.py, argparse_exemplar.py, argparse_lexer.py, argparse_roles.py, cli_usage_lexer.py, and the embedded sphinx_argparse_neo/ directory (~4,400 lines) — all now provided by published packages.
• docs/_static/js/spa-nav.js — Removed; bundled in sphinx-gptheme theme statics.
• tests/docs/_ext/ — Updates imports from old _ext/ paths to sphinx_argparse_neo.* package paths; removes sys.path manipulation from conftest; fixes version assertion in test_sphinx_fonts.py.

Bugs fixed upstream in gp-sphinx (init-2)

During the migration the following issues were found and fixed in gp-sphinx:

• sphinx_argparse_neo/exemplar.py — from argparse_roles import register_roles → from sphinx_argparse_neo.roles import register_roles (would ModuleNotFoundError when installed as a package)
• gp_sphinx/config.py — Removed dead ASSETS_DIR constant (no assets/ dir; spa-nav.js lives in sphinx-gptheme)
• gp_sphinx/config.py — Added html_static_path = ["_static"] and templates_path = ["_templates"] defaults (without these, docs/_static/ was silently skipped — logos, custom CSS, favicons all 404'd)
• gp_sphinx/defaults.py — Added DEFAULT_TOC_OBJECT_ENTRIES_SHOW_PARENTS = "hide" constant

Test plan

• uv run pytest — 1154 passed, 2 skipped
• uv run ruff check . — no issues
• uv run mypy — no issues
• just build-docs — build succeeded (101 pre-existing autodoc/cross-ref warnings, unrelated to this change)
• Visual check via Playwright — logo, sidebar, IBM Plex fonts, argparse code blocks, projects sidebar all rendering correctly

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.96%. Comparing base (097f617) to head (73e19a3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1033      +/-   ##
==========================================
+ Coverage   81.02%   81.96%   +0.93%     
==========================================
  Files          28       28              
  Lines        2630     2545      -85     
  Branches      492      485       -7     
==========================================
- Hits         2131     2086      -45     
+ Misses        368      328      -40     
  Partials      131      131              

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