docs: add research domains and resource registry

- add local external resources registry
- add research domain contract and per-repository research domains
- add raw fact snapshot governance and validation scripts

Validation:
- make test
- git diff --check HEAD~1..HEAD

Note:
- welcome workflow failed because its action input names are stale; core content CI passed.

assets/external-resources/archive.yml

@@ -0,0 +1,3 @@
+version: 1
+description: 已下线、失效、合规风险过高或被替代的外部资源记录。
+resources: []