docs: add env.py guide and update naming convention docs

[AlexeyShalaev]

Summary

• Add new guide/env-py.md page explaining how to configure Alembic's env.py to work with alembic-gauntlet: injected connection / target_schema attributes, minimal working example, SET LOCAL vs SET danger callout, advisory locks, include_object filtering, credentials loading
• Update guide/configuration.md: add missing allowed_fk_prefixes, allowed_check_*, allowed_uq_*, allowed_pk_* attributes; add section on auto-deriving rules from MetaData.naming_convention with resolution order table
• Update guide/quickstart.md: replace outdated defaults list with a full table covering all constraint types; add mention of auto-derive and link to env.py guide
• Add Configuring env.py entry to zensical.toml nav

[codecov]

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