docs: cross_package 'Extending the rule set' section is stale after config refactor

[egouilliard-leyton]

Context

Discovered while reviewing the orphan_detection implementation (issue #17).

Description

docs/arch-policies.md section 2 ("Extending the rule set") still instructs users to edit arch_check.py and append tuples to a CROSS_PACKAGE_PAIRS constant:

CROSS_PACKAGE_PAIRS = [
    ("twenty-front", "twenty-server"),
    ("packages/docs", "packages/twenty-server"),
]

That constant no longer exists — it was removed when cross_package was moved to .arch-policies.toml config. Readers following the docs will look for a symbol that isn't there. The correct approach is now:

[policies.cross_package]
pairs = [
  { importer = "apps/web",    importee = "apps/api" },
  { importer = "packages/docs", importee = "packages/server" },
]

Suggested approach

Replace the "Extending the rule set" subsection with TOML examples matching the current .arch-policies.toml schema. Remove all references to CROSS_PACKAGE_PAIRS as a Python constant.