models: Add comprehensive docstrings to public API types

- Provided detailed docstrings for all properties, methods, and fields
  within public models to enhance clarity and usability of the API.
- Added type-level descriptions for key API response types and utilities.
- Improved coverage of metadata details, structured data, and edge cases
  for PDF processing operations.

Assisted-by: Codex

Author: datalogics-kam

docs/api-reference.md

@@ -3,3 +3,11 @@
 ## Package
 
 ::: pdfrest
+
+## Public Types
+
+::: pdfrest.types
+
+## Public Models
+
+::: pdfrest.models