Ignore link-checker 403s from O'Reilly, SAP Community, Medium and Art of Community (#943)

These sites reject the link checker's automated requests with 403 while loading fine in a browser.
They fail the scheduled Link Check on main and block unrelated PRs such as #942.


Claude-Session: https://claude.ai/code/session_019wZvXn8ovXwaV5W4WQ7H9G

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: rrrutledge

.lycheeignore

@@ -16,4 +16,9 @@ https://github.com/trieshard/source-strategy-assessment/blob/master/framework.md
 https://dl.acm.org/doi/10.5555/3158161.3158175
 https://retromat.org/en/
 # O'Reilly returns 403 to the link checker's automated requests, though the page loads fine in a browser.
-http://www.oreilly.com/pub/e/3884
+http://www.oreilly.com/pub/e/3884
+# These sites return 403 to the link checker's automated requests, though the pages load fine in a browser.
+https://www.oreilly.com/library/
+https://community.sap.com/
+https://medium.com/
+artofcommunityonline.org