Expose utility from swingset/meta to get document list based on type and category

[BRKalow]

Internally, swingset distinguishes between component documentation and standalone documentation. This is evident on the generated metadata objects for each detected document:

  "text": {
    "__type": "component", // 👈 __type property represented here
    "frontmatter": {},
    "category": "default",
    "filepath": "...",
    "relativePath": "components/text/docs.mdx",
    "normalizedPath": "text",
    "componentPath": "components/text",
    "slug": "text"
  },

We should expose a method to get metadata based on the document type and category. This will allow themes to build a navigation element that has access to all available paths.