docs: add lance skills as user guide

[Xuanwo]

This PR adds a basic lance skill as user guide.

Users can install them via

npx skills add lance-format/lance

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Parts of this PR were drafted with assistance from Codex (with gpt-5.2) and fully reviewed and edited by me. I take full responsibility for all changes.

[github-actions]

PR Review

Thanks for adding the Lance user guide skill! This is a useful addition for helping code agents assist Lance users.

P1 Issues

Deprecated API usage - The documentation and example script use num_partitions which is deprecated (see python/python/lance/dataset.py:2754). Consider updating to use target_partition_size instead, or at minimum add a note that num_partitions is deprecated.

Affected locations:

• SKILL.md line 140: num_partitions=256
• references/index-selection.md line 39: recommends num_partitions
• scripts/python_end_to_end.py line 55: uses num_partitions

Minor Suggestions (non-blocking)

• The valid vector index types are: IVF_FLAT, IVF_PQ, IVF_SQ, IVF_HNSW_FLAT, IVF_HNSW_PQ, IVF_HNSW_SQ, IVF_RQ. The list in index-selection.md is mostly accurate but doesn't include IVF_HNSW_FLAT which might be useful for users who want exact search within HNSW clusters.

Overall the skill content is well-structured with clear workflow decision trees and troubleshooting patterns.

[jackye1995]

we are missing a few here like label list, bloom filter, rtree. Should also mention how to handle json data index.