Add Markdown .md files to the default list of documents for RAG

Add .md extension to files that are read for RAG. --- For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/NVIDIA/ChatRTX/pull/117?shareId=a5b1b57a-0d64-4af6-b8ce-087919d1697f).