fix: add HF Spaces frontmatter to root README

HF Spaces reads the config from the root README.md, not spaces/README.md.
Adding the required frontmatter block so the Space can detect the sdk,
app_file (spaces/app.py), and other metadata.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -1,3 +1,15 @@
+---
+title: Doc Ingestion RAG Demo
+emoji: 📚
+colorFrom: blue
+colorTo: indigo
+sdk: streamlit
+sdk_version: "1.37.0"
+app_file: spaces/app.py
+pinned: false
+license: mit
+---
+
 # Doc-Ingestion
 
 Citation-aware RAG system for ingesting documents and generating grounded answers with truthfulness scores — CLI, API, and Streamlit UI.