feat(docs): Add Plausible analytics to Sphinx docs

[ericapisani]

Sphinx docs had no usage analytics, so there was no visibility into which API reference pages get traffic. Overrides the shibuya theme's empty extra-head partial to load the Plausible script.

Fixes PY-2757
Fixes #7318

[linear-code]

PY-2757

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Codecov Results 📊

✅ 121930 passed | ⏭️ 6762 skipped | Total: 128692 | Pass Rate: 94.75% | Execution Time: 430m 18s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ No test changes detected

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2489 uncovered lines.
✅ Project coverage is 90.29%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    90.29%    90.29%        —%
==========================================
  Files          193       193         —
  Lines        25621     25621         —
  Branches      9438      9438         —
==========================================
+ Hits         23132     23132         —
- Misses        2489      2489         —
- Partials      1436      1436         —

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