feat(kg): add markdown export format for knowledge graph

- Add kg_ocr/export/markdown_exporter.py with export_markdown()
- Cluster entity co-occurrences using Louvain community detection
- Wire -f markdown into `ocr-pipeline kg export` CLI command with KG_REPORT.md default
- Add unit and CLI tests in tests/test_graph.py
- Document markdown export in README.md
- Untrack pycache binaries and ignore graphify-out and KG_REPORT.md

.gitignore

@@ -80,4 +80,6 @@ Thumbs.db
 # Project specific
 data/
 notebooks/
-.env
+.env
+graphify-out/
+KG_REPORT.md