docs: Document data structures

[Copilot]

Users had no way to know what fields and types to expect from the database before using the library — no reference existed for the output data model.

Changes

• site/docs/data-structures.md (new) — field-level reference for every table MarkdownDB populates:

  • files — _id, file_path, extension, url_path, filetype, metadata with types, descriptions, and a full JSON example
  • tags — name
  • file_tags — join table columns with foreign-key notes
  • links — link_type, from, to
  • tasks — all 9 fields including inline-metadata fields (due, completion, created, start, scheduled, list, metadata) with a JSON example
  • ASCII schema diagram showing table relationships and cascade behaviour

• site/docs/index.md — adds link to the new page under Guides

Example output documented

{
  "_id": "a3f5c2d1e4b6...",
  "file_path": "/content/blog/hello-world.md",
  "extension": "md",
  "url_path": "blog/hello-world",
  "filetype": null,
  "metadata": {
    "title": "Hello World",
    "tags": ["news", "tutorial"],
    "draft": false
  }
}