refactor: DocsIndexGrid and DocsIndexList duplicate the same DOCS_GROUPS grouping logic

[yakew7]

Problem

src/components/docs/docs-index-grid.tsx and src/components/docs/docs-index-list.tsx (the two /docs index view modes added in #205) both independently implement the exact same grouping logic:

// docs-index-grid.tsx
{DOCS_GROUPS.map((group) => {
  const entries = docsPages.filter((entry) => entry.group === group);
  if (entries.length === 0) return null;
  return ( /* ...grid render... */ );
})}

// docs-index-list.tsx
{DOCS_GROUPS.map((group) => {
  const entries = docsPages.filter((entry) => entry.group === group);
  if (entries.length === 0) return null;
  return ( /* ...list render... */ );
})}

The DOCS_GROUPS.map(...).filter(entry => entry.group === group) grouping/skip-empty logic is character-for-character identical in both files, only the JSX each renders per group differs. If a third view mode is ever added (or the grouping rule changes, e.g. to also skip a group behind a feature flag), it has to be updated in two places that can silently drift.

Fix

Extract a small shared helper, e.g. groupDocsByTopic(docsPages: DocsNavEntry[]): { group: DocsGroup; entries: DocsNavEntry[] }[] in src/lib/docs-nav.ts, that both components call and then just map over to render their own JSX per entry.

Acceptance criteria

• Grouping logic exists in exactly one place
• DocsIndexGrid and DocsIndexList render identically to before
• src/components/docs/docs-index.test.tsx still passes