Modernize Storybook component docs

[fredvisser]

Summary

Modernize the Nimble Storybook component documentation pages with a repository-owned responsive layout, explicit table of contents, reusable previews/showcases/guidance blocks, and token-backed light/dark styling.

The Button docs now include the cropped, inline theme-aware anatomy SVG and stable section metadata. Select and related docs use the same layout conventions. Storybook's public Docs blocks remain responsible for Canvas, Controls, Title, and Meta; custom table-descendant styling was removed to keep the API surface on stock Storybook behavior. The private syntax-highlighter API and unused dependency were removed, and generated Storybook selector workarounds were not expanded.

Validation

• npm run lint -w @ni-private/storybook
• npm run build -w @ni-private/storybook
• git diff --cached --check
• Visual smoke checks for Button in dark mode and Select at a narrow viewport; both rendered without horizontal overflow.

The existing generated-selector loading-skeleton workaround in packages/storybook/.storybook/preview.css is unchanged and remains outside this PR's staged payload.