docs: add FTS v2 migration note

[BubbleCal]

Summary

Docs-only follow-up to #7512.

This adds a migration guide note for the FTS / inverted index format default changing from v1 to v2. The note explains who is affected, how to keep creating v1 indexes with format_version=1, and the compatibility boundary for existing v1 indexes and older Lance readers.

Validation

• git diff --check -- docs/src/guide/migration.md
• cd docs && uv sync --all-extras
• cd docs && uv run mkdocs build
• cd docs && uv run mkdocs-linkcheck src

mkdocs build completed successfully with existing warnings about unrelated missing documentation links outside docs/src/guide/migration.md.