[Customer account UI extensions 2025-07]: Set up component sidebar

[mcvinci]

This PR:

• Adds .doc.ts files and examples for ~60 shared checkout components (plus Chat and StyleHelper) to the customer account surface, so component reference pages are generated directly instead of linking out to checkout
• Migrates the doc build pipeline from build-docs.sh to build-docs.mjs with parallel doc generation and proper type resolution
• Creates build-doc-shared.mjs (utility module required by build-docs.mjs) and helper.docs.ts for the customer-account surface (fixing ~13 component docs that failed to compile due to a missing import)
• Passes the full checkout source as --typesInput and adds React types for reference docs, with surface blanking during builds to avoid duplicate type conflicts
• Removes the empty category overview page so subcategories render directly from component entries
• Aligns subcategories across shared component docs to match the new sidebar IA (e.g., "Feedback" → "Feedback and status indicators", "Interactive" → "Media and visuals")
• Relates to https://github.com/shop/world/pull/486452
• Partially closes https://github.com/Shopify/shopify-dev/issues/69153

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory. If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG. If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.