Update all docs dependencies to latest

[AmitMY]

Summary

Batch-replaces the 10 open dependabot PRs (#207, #209, #212, #214, #220, #221, #235, #236, #237, #238) touching /docs.

The docs requirements were a 110-line pip-freeze from 2023 that pinned every transitive dependency — which is why dependabot kept filing CVE bumps one package at a time. This PR:

• Reduces docs/requirements.txt to the ~13 top-level packages the build actually needs (Sphinx toolchain + the libraries pose_format imports during autodoc), with >= lower bounds at the verified versions. Read the Docs now always installs latest, and dependabot has no pins left to bump in /docs, so this class of PR stops entirely.
• Bumps the Read the Docs Python from 3.8 → 3.12 (current torch/tensorflow don't support 3.8).
• Drops the display_version theme option removed in sphinx-rtd-theme 3.x.

Test plan

• Fresh venv (Python 3.12) with the new requirements resolves cleanly at latest: Sphinx 9.1, torch 2.13, tensorflow 2.21, numpy 2.5, sphinx-rtd-theme 3.1
• sphinx-build -b html docs succeeds; API pages render with content
• Remaining warnings are pre-existing (stale pose_format.pose_test rst pointing at the moved test module, duplicate-object cosmetics) — not dependency-related

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