Doc: update core file system diagram + add fork diagram

doc/core_file_system.pu

@@ -8,9 +8,9 @@ skinparam sequence {
 }
 
 participant "System" as S
-participant "Folder 3\n /A.json" as F3
-participant "Folder 2\n /B.json" as F2
-participant "Folder 1\n /A.json\n /C.json" as F1
+participant "Folder of block **B5_a**\n /A.json" as F3
+participant "Folder of block **B4**\n /B.json" as F2
+participant "Folder of block **B3**\n /A.json\n /C.json" as F1
 
 == Direct READ ==
 

fork.pu

+@startuml
+
+(B5_b) as B5f #LightBlue
+(B6_b) as B6f #LightBlue
+
+skinparam usecase {
+	BackgroundColor DarkSeaGreen
+	BorderColor DarkSlateGray
+	ArrowColor Blue
+}
+
+(B3) -> (B4)
+(B4) --> (B5_a)
+(B5_a) -> (B6_a)
+(B6_a) -> (B7_a)
+(B4) -[#LightBlue]> (B5f)
+(B5f) -[#LightBlue]> (B6f)
+
+@enduml
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