[#1097] docs: Describe authentication mechanisms

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

docs/images/authentication/authoverview.puml

@@ -0,0 +1,28 @@
+@startuml authoverview
+!include <c4/C4_Container.puml>
+!include <c4/C4_Component.puml>
+AddElementTag("smart-contract", $bgColor=#0abab5)
+
+Person(user, "User", "User with private key")
+
+Container_Boundary(stor, "FrostFS Storage") {	
+	Component(verify, "Sign Service", $descr="Check request signature")
+	Component(apesvc, "APE Service")
+	Component(objsvc, "Object service")
+}
+
+Container_Boundary(neogo, "Blockchain") {
+    Interface "NeoGo"
+    Component(ffsid, "FrostFS ID", $tags="smart-contract", $descr="Stores namespaces and users")
+    Component(policy, "Policy", $tags="smart-contract", $descr="Stores APE rules")
+}
+
+Rel_R(user, verify, "Requests", "gRPC")
+Rel_R(verify, apesvc, "Access control")
+Rel_R(apesvc, objsvc, "Operation")
+Rel_D(apesvc, NeoGo, "Get data to validate request")
+Rel("NeoGo",  ffsid, "Fetch users")
+Rel("NeoGo",  policy, "Fetch policies")
+
+SHOW_LEGEND(true)
+@enduml