Update arXiv section to focus on OAI-PMH API and add data format details

Author: Opsmithe

sources.md

@@ -13,22 +13,18 @@ and systems science, and economics. All arXiv articles are available under
 various open licenses or are in the public domain.
 
 **API documentation link:**
-- [arXiv API User Manual](https://info.arxiv.org/help/api/user-manual.html)
-- [arXiv API Reference](https://info.arxiv.org/help/api/index.html)
 - [arXiv OAI-PMH Interface](https://info.arxiv.org/help/oa/index.html)
-- [Base URL (Standard API)](https://export.arxiv.org/api/query)
 - [Base URL (OAI-PMH)](https://oaipmh.arxiv.org/oai)
 - [arXiv Subject Classifications](https://arxiv.org/category_taxonomy)
 - [Terms of Use for arXiv APIs](https://info.arxiv.org/help/api/tou.html)
 
 **API information:**
-- No API key required for either interface
+- No API key required
 - Query limit: No official limit, but requests should be made responsibly
-- **Standard API**: Data available through Atom XML format, supports search by
-  various fields
+- **Data format**: OAI-PMH XML format with structured metadata fields
 - **OAI-PMH Interface** (used by `arxiv_fetch.py`):
   - Structured metadata harvesting with resumption tokens
-  - Better license metadata extraction for CC-licensed papers
+  - License information extracted from `{http://arxiv.org/OAI/arXiv/}license` XML field
   - Recommended 3-second delays between requests
   - Supports date-based filtering for bulk harvesting
 - Metadata includes comprehensive licensing information for each paper