docs: fix Cohere dataset links

[charles-openclaw]

Summary:

• Replace the stale Cohere Wikipedia dataset link in the README with Cohere's public embedding archive explainer.
• Update the matching Cohere 768-dimensional dataset link in the Streamlit welcome copy so the app text does not keep pointing at the unavailable Hugging Face dataset page.

Verification:

• Reproduced the existing README link returning HTTP 401 for https://huggingface.co/datasets/Cohere/wikipedia-22-12/tree/main/en.
• Confirmed the replacement URL returns HTTP 200 with curl.
• Ran python3 -m py_compile vectordb_bench/frontend/components/welcome/explainPrams.py.
• Ran a static assertion that both edited files no longer contain the stale Hugging Face URL and do contain the Cohere archive link.
• Ran git diff --check.

Fixes #788

[sre-ci-robot]

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