feat: Add rag url knowledge examples

[fangyinc]

Add rag url knowledge examples.

To test the current dbgpts, you need to rely on the code of https://github.com/eosphoros-ai/DB-GPT/pull/1341 See workflow/rag-save-url-to-vstore/README.md and workflow/rag-url-knowledge-example/README.md

[dexuandai]

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