docs: document step.authz_add_policy, step.authz_remove_policy, step.authz_role_assign

[intel352]

Problem

The plugin.json manifest declares four step types, but the README only documents step.authz_check. The following steps are undocumented:

• step.authz_add_policy — adds a policy rule at runtime
• step.authz_remove_policy — removes a policy rule at runtime
• step.authz_role_assign — adds/removes role mappings at runtime

These are essential for multi-tenant applications that need to provision authorization policies dynamically when new tenants are onboarded.

Proposed Solution

Add documentation sections for each step type with config examples, including template-based rule definitions.