Adding PlainID integration documentation

src/oss/python/integrations/providers/plainid.mdx

@@ -0,0 +1,15 @@
+---
+title: PlainID
+---
+
+[PlainID](https://www.plainid.com/) provides policy-based authorization (PBAC) and centralized policy enforcement.
+With LangChain, you can integrate PlainID checks to ensure only authorized users or tenants can invoke tools
+or access RAG data in your LLM applications.
+
+## Installation and Setup
+
+<CodeGroup>
+```bash pip
+pip install langchain_plainid
+uv add langchain_plainid
+</CodeGroup>