feat: add introduction to requirement structure

docs/security/requirements.mdx

@@ -4,6 +4,12 @@ sidebar_position: 4
 
 # Requirements
 
+The requirements in this section are organized hierarchically, with top-level requirements defining
+the core rules and obligations that must be met, serving as broad objectives for Bitwarden's
+security model. Sub-requirements expand on these by addressing specific scenarios, exceptions, or
+clarifications, and may override their parent requirement when explicitly stated. Sibling
+requirements at the same level must remain independent and free of contradictions.
+
 ## Vault data (VD)
 
 :::warning Draft - Early version