docs: split model source records and unify project instructions (#113)

* docs: split model sources and consolidate project instructions

* docs: normalize empty model navigation footer

docs/model-sources/providers/xai/README.md

@@ -0,0 +1,20 @@
+# xAI：来源导航
+
+[总索引](../../README.md) · [官网证据与边界](SOURCES.md)
+
+本目录包含 7 个单模型文件、2 个单接入面文件。核验日期与状态来自迁移前记录。
+
+## 接入面
+
+- [compute/providers/xai.json](access/providers--xai.md)
+- [compute/model-specs/xai.json](access/model-specs--xai.md)
+
+## 模型
+
+- [`grok-4-1-fast-reasoning`](models/grok-4-1-fast-reasoning.md)
+- [`grok-4-3`](models/grok-4-3.md)
+- [`grok-4.20-0309-reasoning`](models/grok-4.20-0309-reasoning.md)
+- [`grok-4.3`](models/grok-4.3.md)
+- [`grok-4.5`](models/grok-4.5.md)
+- [`grok-4.7`](models/grok-4.7.md)
+- [`grok-build-0.1`](models/grok-build-0.1.md)