docs: clarify transform() behavior for indexes, views and triggers

[princethummar1]

Closes #849

Summary of Change

Updates the overview paragraph in the Custom transformations with .transform_sql() section of the Python API documentation to accurately reflect the current behavior of .transform():

• Indexes: Preserves and recreates indexes where possible (rather than stating they are not upgraded).
• Views: Cross-references the Tables referenced by views section explaining that referenced tables are safely transformed without breaking view definitions.
• Triggers & Column Changes: Clarifies that view definitions and triggers referencing renamed or dropped columns must still be manually updated.

Verification

• Ran cog -r docs/*.rst
• Ran flake8 and mypy sqlite_utils (clean pass)
• Ran pytest tests/test_docs.py tests/test_transform.py (212 passed)

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📚 Documentation preview 📚: https://sqlite-utils--851.org.readthedocs.build/en/851/