Clarify transform() behavior for indexes, views, and triggers

[ParsingMAINFRAME]

Refs #611.

The custom-transform documentation says transform() does not automatically
"upgrade" indexes, views, or triggers. That no longer explains the behavior:
indexes have been recreated since 3.38, and simple indexes on renamed columns
are supported in 4.2.

This documentation-only change explains the differences on current main:

• Explicit indexes are recreated, with simple column references renamed.
  It describes which index cases raise TransformError and when to recreate
  indexes manually.
• Views keep their definitions, with a link to the existing explanation.
• Triggers on the original table are dropped, or remain on the retained
  table when keep_table is used. Triggers attached elsewhere are unchanged.

It also explains how to append CREATE TRIGGER statements to transform_sql()
when renaming a column, and removes wording suggesting stale view references
are inherent to SQLite views. transform(rename=...) rebuilds the table;
SQLite's native ALTER TABLE ... RENAME COLUMN has different behavior.

Validation on macOS, Python 3.14.7 / SQLite 3.53.4, base 6bc1d33:

• pytest: 1497 passed, 16 skipped.
• transform tests with --sqlite-autocommit: 112 passed.
• Sphinx HTML build with warnings as errors, Black, flake8, mypy, Pyright,
  ty, Cog, and codespell passed.
• Synthetic in-memory examples inspected generated SQL and sqlite_master
  before/after, including trigger loss, keep_table, stale references, index
  rejection, and rollback after a failed unique-index recreation.
• The CLI check without dev dependencies passed, including clean isolated
  editable builds of both unchanged main and the patched source. An initial
  editable-install failure was traced to a local cached .pth file carrying
  macOS's hidden flag, which Python skips; it also failed on unchanged main
  using that same affected cache artifact.

Small example of the trigger behavior:

from sqlite_utils import Database

with Database(memory=True) as db:
    db.executescript("""
        CREATE TABLE thread (subject TEXT);
        CREATE TABLE audit (value TEXT);
        CREATE TRIGGER subject_changed AFTER UPDATE OF subject ON thread
        BEGIN INSERT INTO audit VALUES (NEW.subject); END;
    """)
    print([trigger.name for trigger in db.triggers])  # ['subject_changed']
    db["thread"].transform(rename={"subject": "title"})
    print(db.triggers)  # []

AI assistance: OpenAI Codex helped investigate the behavior, draft the wording,
and run the checks.

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📚 Documentation preview 📚: https://sqlite-utils--896.org.readthedocs.build/en/896/