Fix basePath issues in Doctocat UI components

[rezrah]

Towards primer/brand#1059

Fixes 3 bugs:

1. Index cards now resolve URLs with basePath, previously 404s otherwise
2. Index card images now resolve frontmatter images correctly in basePath contexts
3. Forwards basePath to Doctocat UI (which can't access it otherwise) and applies it to ReactCodeBlock previews to fix 404'ing images

[Copilot]

Pull Request Overview

This PR ensures that the Next.js basePath is respected by Doctocat UI elements (index cards and live code previews) by exposing it via environment, wiring it through context, and rewriting URLs accordingly.

• Expose basePath from Next.js config to Doctocat via NEXT_PUBLIC_DOCTOCAT_BASE_PATH.
• Forward basePath into React context and apply it to index card links.
• Add codeTransformer and hook it into ReactCodeBlock so previewed images get the correct path.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/theme/doctocat.config.js	Expose basePath to Doctocat via env var
packages/theme/components/layout/root-layout/index.tsx	Provide basePath in ConfigContext
packages/theme/components/layout/index-cards/IndexCards.tsx	Wrap cards in Next.js Link and prepend basePath to links
packages/theme/components/layout/code-block/code-transformer.ts	Utility to rewrite image src with basePath
packages/theme/components/layout/code-block/ReactCodeBlock.tsx	Apply codeTransformer to live previews via transformCode
packages/theme/components/context/useConfig.tsx	Add basePath field to ConfigContextValue
.changeset/warm-teachers-repeat.md, .changeset/chatty-trains-explode.md	Document patch updates for basePath support

Comments suppressed due to low confidence (3)

packages/theme/components/layout/index-cards/IndexCards.tsx:95

• [nitpick] Using a placeholder href="#" may introduce unintended URL fragments; consider removing the href prop on Card and relying on Next.js Link's passHref behavior, or render Card as via the as prop.

              <Card href="#" hasBorder>

packages/theme/components/layout/index-cards/IndexCards.tsx:94

• Add unit or integration tests to verify that index card links are correctly prefixed with basePath when NEXT_PUBLIC_DOCTOCAT_BASE_PATH is set.

            <Link legacyBehavior passHref href={item.route}>

packages/theme/components/layout/code-block/code-transformer.ts:1

• Introduce unit tests for codeTransformer to ensure it properly prepends basePath to various local image src patterns and skips external URLs as intended.

export const codeTransformer = (sourceCode: string, basePath: string): string => {