docs: expand the FTS index doc explaining the training process and multiple partitions

[westonpace]

No description provided.

[github-actions]

ACTION NEEDED
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[github-actions]

Code Review

This is a documentation-only PR adding explanations of the FTS index training process, configuration, and distributed training. I verified the documented values against the implementation.

Technical accuracy check:

• ✅ Environment variable defaults match code: LANCE_FTS_NUM_SHARDS (num compute CPUs), LANCE_FTS_PARTITION_SIZE (256 MiB), LANCE_FTS_TARGET_SIZE (4096 MiB)
• ✅ Fragment mask formula (fragment_id as u64) << 32 matches code comment at builder.rs:106
• ✅ Per-partition metadata filename pattern part_<id>_metadata.lance matches code at builder.rs:1102
• ✅ skip_merge behavior accurately described

No P0/P1 issues found. The documentation is accurate, well-organized, and provides valuable operational guidance for users configuring FTS indexes at scale.

LGTM 👍

[prrao87]

LGTM!

Could you also please add a note in the quickstart page on FTS pointing to this page? This page is buried under the scalar index page and might be hard to find unless a user specifically searches for FTS (in which case they might stumble across the quickstart page, so it's best to lead the reader here for advanced usage).

At the end of quickstart/full-text-search/, we could add an H2 header with the following text:

## Advanced usage

For advanced usage instructions with different tokenizers and more technical details on the index training process, visit the [full-text index](../format/table/index/scalar/fts/) section.

[westonpace]

Could you also please add a note in the quickstart page on FTS pointing to this page?

@prrao87 done. I also fixed a number of broken links.