Fix dead Jina embeddings documentation links

[auroraxo]

Summary

Replace two references to a removed Jina AI announcement with the canonical Hugging Face model card for jina-embeddings-v2-base-en.

The old URL now returns HTTP 404; the model card returns HTTP 200 and preserves the specific 8K-context model information those sentences cite.

Verification

• Confirmed both old references return 404
• Confirmed the replacement URL returns 200
• Documentation-only change; no runtime behavior changed

[auroraxo]

Context on how I found these: I ran a documentation link-rot scan over simonw/llm@main (31 docs files, 831 outbound links checked) and reviewed the results by hand before opening this PR.

Two other findings from that scan that I did not touch here, because they need a maintainer decision rather than a mechanical fix:

• docs/logging.md:438 — the BLAKE2b link is https://en.wikipedia.org/wiki/BLAKE_(hash_function (missing the closing paren, so it 404s). The intended target https://en.wikipedia.org/wiki/BLAKE_(hash_function) returns 200. Markdown's handling of the trailing ) is the reason it got truncated; <...> angle brackets fix it.
• docs/fragments.md:100 — the sample output references https://raw.githubusercontent.com/simonw/llm-docs/refs/heads/main/llm/0.22.txt, which is now 404 (the llm/ tree is gone from simonw/llm-docs). It is illustrative output rather than a live link, so it may be intentional.

Happy to fold either of those into this PR if you'd like them fixed the same way.