docs: fix broken observability and tokenizer links

[Xuanwo]

Summary

Fixes the two documentation 404s reported in #8364.

• docs/src/guide/tokenizer.md: jieba-rs moved its dictionary into the jieba/ crate path (jieba/src/data/dict.txt).
• docs/src/guide/observability.md: lance::metrics is feature-gated and docs.rs currently builds with no features, so docs.rs/.../lance/metrics/ 404s. Point the guide at the in-repo metrics.md source that the page already includes, and enable the metrics feature in [package.metadata.docs.rs] so the module is published after the next crate release.

Test plan

• Confirmed both replacement URLs return HTTP 200
• Scheduled docs link checker should clear the Errors entries for these two files once this lands on main

Fixes #8364

  

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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