docs: convert ingestion module docstrings to Numpydoc standard - #1193

Summary

Converts all public method docstrings in graph_builder.py to follow the Numpydoc parameter and return listing standard.

Changes

• Added Parameters/Returns sections to 13 docstrings
• Expanded class and module-level docstrings with descriptions
• Converted inline 'Args:' sections to proper Numpydoc format
• No logic changes, documentation only

Checklist

• File compiles clean (py_compile)
• No logic changes
• Follows existing Numpydoc convention

Fixes #1108