Add collapsible styles for C++ docs

[svekars]

• Add collapsible "Subclassed by" list support to the theme
  • Paragraphs starting with "Subclassed by" that contain more than 5 links are automatically collapsed to a single line with a
    "See All" toggle button. Example C++ docs: https://docs-preview.pytorch.org/pytorch/pytorch/174096/cppdocs/api/optim/index.html#_CPPv4N5torch5optim9OptimizerE
  • Moved CSS/JS from pytorch/docs/cpp/source/_static/ into the theme so it's available to all docs, not just C++ docs

Test plan

• Build the C++ docs and verify "Subclassed by" sections with >5 links show collapsed with a "See All" button
• Click "See All" to expand, "Hide" to collapse
• Verify sections with ≤5 links are not affected

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