Display document hierarchy as a tree structure.

By default, shows root documents (entry points with no incoming block references)
and their children organized in a tree format. Each level of nesting represents
a block reference relationship between documents.

Use -k/--key to start the tree from specific document(s), regardless of whether
they are roots. This is useful for viewing subtrees or documents involved in
circular references that have no natural root.

Output formats:
  markdown  Nested list with links (default): - [Title](key)
  keys      Document keys only
  json      JSON array with nested structure

Use -d/--depth to limit how deep to traverse (default: 4).