feat(docs-site): swizzle DocCard, restyle to match LinkCard

[aaronlee777]

Summary

Swizzle `DocCard` and `DocCardList` from `@docusaurus/theme-classic` so we own the markup and the styling, then restyle the CSS modules to mirror the `LinkCard` look from #2102 (closed) — bringing those design choices into the components Docusaurus uses for ``.

Card styling

• 16px radius, 2rem padding
• Hairline 1px `#ebedf0` border, no shadow
• Hover: border bumps to `#d0d3d8`
• Dark mode: `rgba(255,255,255,0.03)` bg, `rgba(255,255,255,0.1)` border (hover to `0.22`)

Typography

• Title: 1.25rem / 600 / `-0.01em` letter-spacing, `#1b1b1d` (white in dark)
• Description: 0.95rem / line-height 1.6, `#525860` (white-65 in dark)

React structure of the swizzled components left unchanged. The existing Infima `row` + `col col--6` grid in `DocCardList` continues to drive layout — no extra grid added.

Heading tokens

Also bumps `--ifm-heading-color` to pure black in light mode and pure white in dark mode (was inheriting Infima's soft `#e3e3e3` on dark).

Test plan

• Run `docs-site` dev server, open a page that uses `` (e.g. `docs/reference/company/hooks/index.mdx`). Cards should render with the new 16px radius / hairline border / clean typography in both light and dark mode.
• Hover a card and confirm the border darkens in light / brightens in dark.
• Open any docs page and confirm h1/h2/h3 render pure black on light and pure white on dark.

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