Docs(ci): Adopt Sphinx inventories

[tony]

Sphinx projects need discoverable references to the Python documentation on libtmux.org. Pin the docs build and publisher to the rebased inventory producer from libtmux/docs#166. Built versions gain objects.inv, targets.json, footer downloads and Sphinx setup help. Root publication composes objects-full.inv from selected default catalogs.

The producer is rebased onto docs main and retains #157's exact head. It includes durable target identities, migration aliases, canonical Sphinx option aliases and uv-managed Python tooling with Ruff and ty.

Targets docs-site-deploy, the existing libtmux.org publishing integration.

Validation

• Rebased onto 37151aca55f6fcbbda9a73e38c61e3d9b203f7d2. Only .github/workflows/docs.yml changes, with both producer refs set to the same commit. All non-pin content is preserved.
• Actionlint 1.7.12 reports the identical concurrency.queue diagnostics on the pinned base and this branch; no new messages. GitHub documents the field in its concurrency reference.
• At the rebased docs producer, lint, formatting, ty and workspace type checks pass. Focused inventory suites pass 68 tests, with seven assembled-output tests explicitly skipped in source-only mode.
• Fresh local HTTP consumers under Sphinx 8.2.3 and 9.1.0 each pass 3,028 RST and 3,028 MyST references across 16 catalogs. Compiled desktop/phone footer controls and all sixteen exact-byte inventory downloads pass. The prior CI artifact's source tree is identical to the rebased producer.
• The docs outer loop hits the real-corpus resolver test's five-second timeout; that test passes in isolation. This is not an overall outer-loop pass. Native library matrices were not rerun locally for this workflow-only change.

Keep this PR as a draft for review. Update both refs to the landed docs commit before merging, and publish every selected default catalog, including Python stable, before canonical root composition. Newly triggered CI and live publication remain separate from the local checks above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.45%. Comparing base (37151ac) to head (d0b9485).

Additional details and impacted files

@@                 Coverage Diff                  @@
##           docs-site-deploy     #787      +/-   ##
====================================================
+ Coverage             52.37%   52.45%   +0.08%     
====================================================
  Files                    26       26              
  Lines                  3729     3729              
  Branches                747      747              
====================================================
+ Hits                   1953     1956       +3     
+ Misses                 1472     1469       -3     
  Partials                304      304              

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