Add docs

docs/components/button.md

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+# Button
+
+## Import
+
+```ts
+import { Button } from "@tria/ui";
+```
+
+## Variants
+
+Рекомендуемая модель:
+- `data-variant="primary" | "outline" | "ghost" | "danger"`
+- `data-size="sm" | "md" | "lg"`
+
+## Example
+
+```tsx
+<Button data-variant="primary" data-size="md">Continue</Button>
+<Button data-variant="outline" data-size="md">Cancel</Button>
+```
+
+## Design tokens
+
+- `--btn-h-sm/md/lg`
+- `--btn-px-sm/md/lg`
+- `--btn-radius`
+- `--btn-gap`