docs: extract a page to describe policy and FAQ

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diff --git a/doc/share/authz/POLICY.md b/doc/share/authz/POLICY.md
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+# Policy
+
+A policy is a predicate that describes if a subject can perform an action
+against a specific resource.
+
+```ruby
+  policy(:parent) { predicate }
+  policy(:partner) { predicate }
+  policy(:sibling) { predicate }
+  policy(:child) { predicate }
+
+  enable(:permission, on: resource).when { parent | partner }
+```
+
+Authorizaion uses policies to determine if a subject in a specific context is
+authorized to perform an action against a resource.
+
+```ruby
+  def can?(subject, action, resource)
+  end
+```
+
+## Policy Language
+
+A policy language facilitates:
+
+1. the specification of composite policies, which in turn forms the basis of trust delegation.
+1. **the static analysis of policies and system configuration.**
+
+## Example
+
+The following hierarchy will be used as the basis for expression policy.
+
+```ruby
+class Organization
+  has_many :groups
+end
+
+class Group
+  belongs_to :organization
+  has_many :projects
+end
+
+class Project
+  belongs_to :group
+  has_many :issues
+end
+
+class Issue
+end
+```