Fix dropdown width and add MyST directives test page

[svekars]

• Fix sphinx-design {dropdown} directives rendering at only 30% page width instead of full article width
• Add comprehensive test page (test-myst-directives.md) covering all common MyST and sphinx-design
  directives

The issue

.sd-card in _sphinx_design.scss sets width: 30% and max-width: 500px for card grid layouts. Sphinx-design renders dropdowns as .sd-card.sd-dropdown, so dropdowns inherited these card width constraints and appeared as a narrow strip instead of spanning the full content area. See here - scroll down to dropdown.

The Fix

Added .sd-card.sd-dropdown overrides in _sphinx_design.scss that set width: 100% and max-width: 100%, and disable the card hover underline animation on dropdowns. The selector is specific to elements with both classes, so regular cards in grids are unaffected.

Test plan

• Build theme: npx grunt default
• Build docs: cd docs && python3 -m sphinx -b html . _build/html
• Verify dropdowns span full article width on test-myst-directives.html
• Verify existing cards on test-cards.html are unchanged (still 30% width in grids)
• Check dropdown behavior at mobile, tablet, and desktop breakpoints
• Test against pytorch docs: Bump pytorch_sphinx_theme to 0.4.11 pytorch#184425: Preview looks good: https://docs-preview.pytorch.org/pytorch/pytorch/184425/library.html

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