feat: Capture resoning content from Reasoning LLMs like R1, o1
See this note on [reasoning-content](https://langroid.github.io/langroid/notes/reasoning-content/).
Also:
- fix: `DocChatAgent.ingest_docs` and `DocChatAgent.ingest_doc_paths` are passed a `metadata` argument,
the `metadata.source` will now be *appended* to the original document (chunks) `metadata.source`